SELECTIVE mimwnoi?: OF OVA IN THE ADULT OVARY

EDGAR ALLEK, WILLIAM B. KOUNTZ, AND BYRON F. FRANCIS Waslizngtoii Untverszty I\‘cRool o f Mc~tlictsc., St . Louts, aiid Drlpartmeiit of

Anatomy, Uiizccrsaty of Mtssourt

ONE C H A R T A N D OXE PLATE (FIFTEEN FIGURES)

Counts of ova in ovaries at different ages have demoii- strated that relatively few of the total numbers of ova in the mammalian ovary ever attain to maturation and are ovulated‘ (Arai, ’20). The full extent to wliich degeneration of ova occurs is, however, not readily appreciated. The present conception is that all tlie definitive ova are differentiated at birth or surely before the attainment of puberty; that they coiistitute a reserve stock, lying dormant until sexual maturity, when successive groups begin rapid growth follow- ing the maturation of preceding groups. The following statement coricerning the conditions in the sow (Corner, ’21) illustrate this conception : “It appears that mature ovaries of noii-pregnant animals contain a reserve stock of follicles of 5 mm. diameter or less. Oiie or two days before the onset of oestrus some of the follicles rapidly enlarge to the full diameter of 7-10 mm., and the enclosed ova pass through the preliminary stages of maturation. ” I t is usually sup- posed that not many more than a re ultimately ovulated begin tlie above-described preovulation growth.

I n an earlier paper the writer ( ’ 2 3 ) showed that all tllc ova are not present in the mouse ovary at puberty, but that new ones are formed periodically during sexual maturity. This suggested the probability that tlie definitive ova begin

‘Arai has added accurate el-iilence f rom counts of an cxtenrled series of rat Ire diseusses rather fully earlier work ill this field ill t l ~ e hnm:i~l ov-;~ry. ovaries.

44.i THh. . ) I J I h R I C 4 h J O r R h 4 L OF i h \ l O \ ! l , 101.. $4, A 0 ::

tlieir tlifl'cretitiation a t some fairly definite period previous t o tlieir maturation. Siuce many new ova are pci*iotlically atlcletl iii this way, more tliaii has b c w i supposctl must tlog:.c~tierate. This affor(1s ail iiicreasecl opportunity for ~ i g o r o u s selection.

IYliilc it is probal)Ic that the elimination of the greatest tii:ml,ev of ova takes 1,lace in tlie c>arl~- stagc.s of c1evelol)rneiit of the folliclcs, eridcncc is presented in this paper wliicli sliows that the eliminaiioti of coiisiderahle numbers cont innes iiito late stages of follicular growth.

I I X l e as1)iratiiig tlici cotitelits of the f olliclcs f i ~ m ovaries of swiiie f o r tlic cxtraction of the follicular liormotic (Allen aiitl Doisy, '23), it w i s quite evident that oraries containitig small folliclcs liacl many more visible follicles2 per. o \ ~ i r y t l i t i i i iliose cotitniniiig large follicles (compare figs. 1 ant1 2 u-it11 9 to 13). Wlicii a group of large follicles is presetit, 1)ossiil)ly at the sixteciitli to tweiit\--first clay of the cycle, the iicst group due to mature a t the ocstras following are rep- resented hy vcry small follicles which a re scarcely visible on the surface of the ovary (figs. 9 to 13) . Since tlic oestrons cycle iii the sow is of t\\-eiit;v-oiic (lays' duration (Struve, ' I l ; C'oriier, '21) tlic time of maturation of successive generations of follicles is widely separated ; therefore, a majority of thc visiljle folliclcs a t any oiie time are of approximately tlic same size. ('onsequetitly, tlierc is little likeliliootl of c017-

fusing meml)crs of two successive geiicrations. This makcbs i t possible by counting follicles of dificrent sizes in the ovaries of sexuallp matiire animals to ascertain the extent of elimina- tion in the relatively late stages of their gi*o\\-tli; i.e., the last tcw (lays or two weeks of tlicir iiitra-ovariaii life.

1ZlATEHI -\L AS11 VETIIOL)

r i I lie ovaries of s w i i i ~ a re especially favorable material fo r sucli a study for they are large and the growiiig folliclcs protrude from the surface. The rapid witlidra~val of the

' A s poiiitctl out 11y l 'wr l :iiitl Scliuppe ( %l), tlic, iiuiiilwr of surf:ice follicles t o deq-1: iiig f(illic1cs nou l t l lie j i iopurtioii : i l .

f ollicular contents by aspiration into a suction bottle cause+ the immediate collapse of the walls of the follicle. This method therefore assures an accurate count, for it leaves no follicle to be counted twice, and by eliminating those couiited, reveals less evident deep-lging ones which might otherwise have been overlooked.

To obtain differential counts follicles are divided accord- ing to size (average diameter) into four classes: I) Smaller tliaii 3 mm.; 11) 3 to 6 mm.; 111) 6 to I) mm.; IV) larger tliaii 9 mm., aiicl ruptured follicles, represented by red corpora lutea, were listed as class V.

To assure accurate measurement, scales were etched 011

glass slides so tliat tlie follicles were risible tliroiiglt tlie scale.

Ovaries coiitaiiiing follicles smaller than 3 mm. in diameter usually liave siicli a large number of follicles that after count- ing them in sereral ovaries we considered a minimal approx- imatioii (counts hy groups) sufficiently accurate. Actual counts win made in all other classes than I; i.e., follicles larger tliaii 3 mm.

OBSEl<VATIOKS

r i 1 lie counts showing the incideiice and variation in num- ber of follicles a t various stages of development are appended (tables 5 to 8) . I n table 9 are listed counts of newly formed corpora lutea from whicli the average number of definitive ova produced are estimated. These results are summarized in tables 1 and 2.

Ovaries of class I, containing follicles smaller than 3 Dim.

in diameter, may contain as many as severity follicles, all at about the same stage of development (figs. 1 aiid 2, and tables 1, 2, and 5). Fifty-eight per cent of these oraries contain 26 to 43 follicles; 90 per cent contain 21 to 70 follicles. They average 45.5 follicles per ovary.

I n ovaries containing follicles of class TI it is exceptional to find more than X5. Eighty per cent of these ovaries con- tain from 8 to 20 follicles; 96 per cent contain from 8 to 33

448 EllGAli ALLEN, WM. B. X O U S T Z , RYEON F. FRANCIS

100 300 557 100 100

follicles. The average number of follicles per ovary is 15.74 (figs. 3 and 4, and tables 1, 2, and 6).

111 ovaries of class 111, containing follicles from 6 to 9 mm. iii diameter, one rarely counts more than 12 follicles. Ninety per cent of these ovaries contain from 4 to 10 follicles; 99 per cent 3 to 1 2 follicles. The average for this class is 7.07 (figs. 5 to 8, and table 7) .

Ovaries of class IV, containing follicles larger than 9 mm., range from 3 to 12 follicles per ovary; 84 per cent contain 3 to 7 follicles. The average is 5.75 per ovary (figs. 9 to 15, and tahle 8 ) .

4532 4724 "585 575 65 1

TABLE 1

S'irnimary of counts of foZlicZrs of differeiit szzes; the average number in the different classes, and percentage of survizral of follicles computed

upon class I as 100 per cent

45.52 1*5.74

7.07 5.75 6.51

~

I Smaller than 3 nim. 11 3 to A 111111.

111 t i to 9 mm. I\' Larger than 9 nim.

V Iin~~tnredfollicles(redco~~~~raIritelt)

100 34.5 1.5 . .5 12. f; 14.3

1tq)reseiitilig thc average numl)er of definitive ova, those actually ovulated.

The number of ova eliminated or the perceiitage of survival are most accurately figured from the number of the definitive ova produced (class V). This map be best determined by comiting tlie newly formed corpora lutea in a representative group of ovaries (figs. 2, 3, and 7) . The number of definitive ova niidonbtedly varies with the breed of swine and the diet To estimate tlie average for the material from which our counts of follicles were made, we have counted the large red corpora in 100 ovaries takeii at random (table 9). Eighty- one per cent of ovaries contain 4 to 8 corpora per ovary. A i l

average of 6.3 definitive ova per ocstrus is obtained.

TABLE 2

T h e incidence of follicles of different sizes in the ovaries of swine

"

NUMBER 01 FOLLICLES

PER OVARJ

31 33 48 58 TO 5 3 2X

8&W

76-80 71-75 66-70 61-65 5&60 51-55 48 50 41-45 38-40 31-35 26-30

25

81-85

22 21 20 19 78 17 16 15 14 1 3 12 11 1 0 9 8 7 ri 5 4 3 2 1

I

3 2 8 1 8 8

14 1 A 13 15 2 3

i)

1

1

h 1 2 6

1 -5 4 J

I - 8 4

1 5 17 21 14 2.5 21; -. . I ( )

lri l t i 19 1 0 17 14 3 2 4

1

8

12 1 I; 20 18 13

2

1

- 1

~

38 per cent of ovaries contain 26 to 4.5 follicles per ovary. 90 per cent of ovaries contain 21 t o 70 follicles per ovary. 80 per cent of ovaries contain 8 to 20 follicles per ovary. 96 per cent of ovaries contain 8 to 35 follicles per ovary. 90 per cent of ovaries contain 4 to 10 follicles per ovary. 99 per cent of ovaries contain 3 t o 1 2 follielcs per ovary. 84 per cent of ovaries cont:iin 3 t o 7 follicles per ovary. 81 per cent of owries contain 4 t o 8 fo!licles per orary.

149

I)crceiitage of siirvival l~ased upon the clefiiiitive ova ratliei. tliaii upoil 5 . i 5 , the average niim1,er of class IT' fol- licles, is t l i c more accurate. I f class I follicles l ~ c taken as a 1)asis for estimatiiig this pei-ccntagc in classes of progrcs- sivcbly largcr follicles, i t is cvitleiit that by the time the fol- liclc1s attwiii tlic size of 3 to (i mm., only 34.5 per cent arc still 1)rcwiit ; at tlic 6 to 9 mm. stage, 15.5 per cent ; and those actiial1.v oviilatetl, tlic clefinitivo ova, represent only 14.3 per wilt of tlie average iium1)cr of' class I follicles. Tlierefore, % ~ w r c w i t of the follicles o C class I are eliminated before t l i e i~~maii i t ler attain ovulation size. The interval required f o r tliis elimination is probably not longer than one-half to two-thirds tlie leiigtli of the cyzle in swine, or the ten to foni*tcen days preceding ovulation. If the whole life-spaii of the follicle or even that part of i t covered by the duration of o i ie q-cle, instead of just tlie later stages were considered, the percentage of survival woixlcl obviously be much less.

caiinot at prescnt say exactly at what stage of the oc~strous cycle ovaries contaiii follicles of class Such small follicles are not readily visible on the surface of the ovarj- a t ocstrus (figs. 9 aiitl 14 ) , when class I V follicles are a~)pi-oacliiiig riipture size. T t is prohahle that they do not al)l)cai- iiiitil scvcral clays af ter the ovulation of tlic preceding svt. T l i e ovaries of class IT, however, may be quite accurately ~ ~ I ~ i c . c v l a t 6 to 10 (lays before ovnlat io~i .~ Follicles of classes 1 I t mtl TV were iuidouhtedly taken during the period three

Tlie average ni imbc~ of follicles in tlie dificrcnt classes is i)lottctl iii grapli 1.

T I 1 0 fiiicliiig tlrat our avei*agc number of class TV follicles is l c w tliaii tlic averag:.c iiiimlm of clefiiiitivc ova produced per ova1.y (altliougli witliin the limit of error when small groups of ovaricks arc couiitetl) is worthy of comment. The ship-

t o tivc1 (la)-s preceding ovnl a 1' 1011.

r 1 I l i c o\:iiic,s coui i tcd in this w n i k W(TC f i o m :inimnls in wliicli it wis no t ~ ) o s \ i l ~ l c ~ to get tlie oestrous history f o r placing the ovaries clironologically.

" 'l'li(> I c,srilts of 111.. F. 1'. Mc.Kieiizic, of tlie L k p i r t m e n t of Animal lIusl,anclry, 1'1ii1 c i \ t t> of IIissouri, f roin olisc~r\:itioiis of seiernl iiiontlis' clui:itioii upon the l i \ I I I ~ \ ( I \ \ l i c ~ f ( i i ti o l i c ~ : i t ~ o n o r killing, iiiake this statciiieiit ~)ossil)le.

meiit of liogs from ~vliicli the ovaries coiitainiiig corpora werc? takeii may have lmi i above the average fecundity. There is, however, another explaiiatioii t o partly accouiit for this dis- crepancy. The selcctioii of thousands of ovaries containing large follicles from tlie total number collected a t a large

Size mm 3 6 9 12 Class 1 II m m

Gr:ipli 1. The inritleiicc of follicles of various sizes in ovaries of swine.

abattoir has impressed 11s with tlie high incidence of cystic follicles iii the oraries of s ~ i i i e . ~ Sucli an ovary is pictured in figure 15. The caoiitiiinance of elimination during early cystic traiisfoi-mation woultl therefore satisfactorily explain n part of this tliffereiice of 2 per cent. ’ There can I I C no tlouljt :IS to the follicu1:rr origin of these cgs ts (a point

raiscd ljy 1’:ip:iiiicoI:iou : i i i ( I Stockxrd ( ?l), in studies of guinea-pig ovaries), for all iiiteiniediate stages a re represented, 2nd we Ii:i\e been able to isolate apparently nor111:rl O T : ~ f ioin tlir :ispiratrd contents. Fktincts of this iiinterinl injected into spn?ed rats :riitl 1iiic.e gixe positixe tests f o r tlie presence of the f ollicular liormonr ( A411cii and 1)oi.;) ) .

452 EDGAR ALLEN, W R I . B. KOUNTZ, BYRON F. FRANCIS

CONCLUSIONS

In relatively late stages of development a large number of follicles (about 85 per cent) are eliminated from the ovaries of swine. The period covered by these observations prob- ably does not include more than the latter one-half or two- thirds of the cycle, or the ten to fourteen days previous to ovulation. Of the visible follicles a t the beginning of this period, only about 15 per cent attain to full maturation and are ovulated.

DISCUSSION

Comparison of the esteuzt of elirninatio+z of follicles iuz the sow with that in the rat a d mouse from the oiewpoi.nt

of periodic ovogewesis during sexual maturity Although the data presented in this paper deal only with

the elimination of follicles in swine in the late stages of their development, a comparison of conditions in rodents is of interest, because data are available which cover the whole period of life of the ova. Rrai ('20) has contributed some accurate counts by microscopical methods of the total num- ber of ova in both ovaries of a large series of rats at, age intervals from birth to old age. His main conclusions are: a ) the total number of ova in both ovaries decreases rapidly from 35,000 at birth to about 11,000 at twenty days of age; b ) from twenty to sixty days i t remains nearly constant at 11,000 to 10,000 ova; (3) between sixty and seventy days (about the time of puberty) it decreases to 5,000 to 7,000, which number remains fairly constant with only a slight de- crease the first year and a half of life.

Since our data deal with conditions in the sexually mature ovary, let us consider nineteen animals of Arai's series rep- resenting ages from 70 to 559 days. Arai divides the ova into four classes according to size: 1) smaller than 20 I.I in diameter; 2) 20 to 40 p ; 3) 40 to 60 1-1, and, 4) larger tlian 60 1-1. This part of his table is repeated below (table 3).

Totaling his columns and computing an average number a t any period during sexual maturity gives the following results (table 4).

SELECTIVE ELIMINATION OF OVA 453

Since the total number of ova is reasonably constant with only a slight decrease during sexual maturity, these figures represent the condition of the ovary at almost any time dur- ing that period. But in non-pregnant animals an average

TABLE 8

The inciderwe of ova in sezouslly mature animals of Arai's series (albino rat)

NUMBER OF OVA IN BOTH 0VA.RIES AGE-DAY8

Bmdler than 20 c 20 to 40 c 40 to 60 c Laraw t b n 60 c Total

70 80 80 84 95 95

100 100 110 110 140 150 198 206 262 318 385 454 559

6246 8258 4564 9406

10293 6266 6853 5535 5100 7290 8659 8621 2468 7782 3536 5460 4169 4465 4729

197 189 290 177 276 129 133 110 176 307 243 222 164 155 126 126 222 160 107

99 77

179 a5

109 109 43 75 75

101 113 138 100 97 70 87 93 78 37

64 42

145 52 86

110 39 24 4 1 46 62 42 18 32 15 29 18 56 20

6606 8566 5268 9720

10764 6614 7068 5744 5392 7744 9077 9023 2750, 8066 3747 5702 4502 4759 4893

~~ ~~

TABLE 4

Average number of ova of different sizes in both ovaries (Arai's series)

AVERAGE NUMBER PERCENTAGE OF FIRST OF OVA CLASS SURVIVING CLASS

1) Less than 20p ..................... 6300 100 2) 20 to 40p ........................ 184 3 3) 40 to 60,u ........................ 92 1.5 4) Larger khan 60p . . . . . . . . . . . . . . . . . . . 49 0.8

__ ~- _ _ _ ~ -~

of about ten ova mature at each oestrous period, every four to six days (Long and Evans, '22). Also, additional ova are added to the ovary during sexually mature life (Robin- son, '18 ; Arai, 9 0 ; Allen, '23).

Since the ocstrons cycle in tlie rat Iiad iiot becii completely worked out wlicii Arai's work was clone, n correlation of ovo- geiic& with tlie oestrous cycle T \ ' ~ s not possible, and Arai liacl no metliotl of estimating tlict number of ova differ- entiating. It has since lwen shown by the writer that post- p u h r t al orogencsis in the monse is not a continuous process, but a periodic phenomeiion tlcfinitcly related to oestrus. This lias made possiblc tlie estimate t.liat 600 to 1000 new ova may differentiate in the two ovaries of a mouse at each normal oestrous periocl. The oestrous cycles of tlie mouse (Allen, "3.2) anel rat, (Long and Evans, '19-22) are practically identical ; therefore it is probable that results obtained in the mouse are applicalde to tlic rat. To further check this point ti search was made for evicience of the proliferation of the germinal epithelium of a rat ovary taken just after ovula- tioil. This rwulted in a cotwt oJ 154 mitoses itt the germinal

' epitltdittm, tkc. Frst stage of pos tpder ta l ouogmesis. This ~ t . in~bcr co3qiarc.s fazwrcc.ld?j with sinrilar c o t i ~ t s iu wi~oti.sc

war ics at or w a r oestrt.rs. Therefore, a) since the oestrous cycle in the two species is of tlic same length, b ) the same twera.ge number of ova are produced at each ovulation, and, c) orogenesis occurs during sexual maturity to ahout tlic? same degree, tlic applictitiou of results from tlic mouse ovaries to conciitions in tlic rat seems justified.

This provides a iim- viewpoint for the consideratioii of Arai's results. We may coiisicler his tot.al number of ova (tahlc 4) as a sounding of tlie germinal stream at any point in its coiirse betweeii the attainment of pulmty and the ea.rlF meiioptiuse, anel in estimating the rolume flow we may add GOO to 1000 ~ic~ . \ ' ova at iiitcrvtils of fonr to six days and subtract the tea mature? ova that attain preovulation size at the same period. It is cvitleiit that if total numlms of ova, irrespective of their size?, arc coiisiclercd, that tlie part of the germinal stream of tiii iiitliritluwl mouse or rat represented by the two oraricbc; (luring sexual maturity ~ W ~ R C S from large sourccs tluri lig cmlwyoiiic and prepubertal life, receives trilmtaries of fi00 to 1000 atiditioiitil ova crcry fonr to six clays (luring

sexual maturit>-,x and flows into a tlescrt of rigorous selec- tion from which only iiumerical tricldcs periodically emergtl as tlie definitive ova.

Althougli tlierc is grcat diversity of reproductive plienomena in clifferelit mammals, tlierc is little question that the same fundamental principles apply to them all. The search for ovogeiiesis during sexual maturity becomes increasingly difficult with iiicreasecl size of the ovary and length of the oestrons cycle and with a clecrease ill the fecundity of a species. To this reason our lack of more extensive evidence is due. It seems logical, lioxwwr, to regard postpubertal ovogciicsis a s a f untlamental principle applicable to mam- malian reprocluctioii. It is probable, therefore, that in swiiic the average number of class I follicles (less than 3 mm.; 45.52), r e p r e s d i n g as i t does tlie total number at a rela- tively late stage of follicular growth, are already the sur- vivors of considerable selection.

Tlte opt -a t io i t of artificial sdcctio+a o n t h e germ cells

Some of the results of artificial selection upon germ cells are of interest in this connection. Following Stockard's work on the effect of alcohol on the germ plasm, Pearl ('17) adraiiced the hypothesis that alcoliol affects germ cells according to their vitality which is extremely variable. The selective agent proves lethal to the less vigorous, while scarcely or only temporarily iiijuring tlie more vigorous gametes. Danforth ('19) has shown that germ cells are snb- ject to selection on the basis of their genetic potentialities, tlie inlialation of alcohol eliminating more readily gametes h a r i n g factors for ccrtain c*liaracteristics. Since during tlie two weeks hefore ovnlatioii in the normal sow natural sclectioii eliminates considerably more than 80 per cent of tlie ova visible at tlie beginning of this period, the possible significance of this process in maintaining the vigor of tlic species can easily be appreciated.

' In tlie absence of pregnancy.

456 R I G A H ALLEN, W M . R. ICOUSTZ, BYRON F. FRANCIS

l ' h ~ fate of cliiniwatrd ova Tlic question arises at this point as to what becomes of the

large iiumbers of ova which undergo degeneration. I n the rat by far the greatest numbers of o r a are eliminated in the very early stages (Arai, '20, table 4). It is well known that (legenerating ova and atretic follicles are found in consider- able iiumhers in all ovaries and, if more accurate criteria of early degeneration were at hand, it is probable that even larger numbers would he recorded. Balfour ("78) has sug- gested that a 'caiiriibalism' of tlie less vigorous by the more vigorous ova exists as iii the ovaries of some of the inverte- h a t e s . If this applies to the sexually mature mammaliaii 0~7ary, it is accomplished, of course, tlirougli the tissue fluids or prohably hy greater functional demands on the hlootl stream hy the more vigorous OTX. TIE iiitemniediatiou of the iiiterstitial cells in this process is another possibility strik- ingly suggested hy their high i~icideiice in the tlieca of atretic follicles ( Riiigshury, '13).

In this connection the following statement by Burrows ( '23)g coiicerning the growth of cells in tissue cultures seems lwrtineiit : ' ' The outer cclls of the traiisplant grow at tlie c~speiisc of tlie inner ones. Those cells in tlie middle of the fragmeiit from wliicli nut rimelit is withdra~~-ri resemljle cells tlcgeiiernting in the presence of oxygen. Wliile this degenera- tion morphologically resembles that in cells from which osygeii is witliheld, it differs from this in that i t is noii-toxic."

It is possible, then, that the ova which are elimiiiated iiiidergo a nontoxic degeiieration, and, as Robinson has sng- gcstecl, that the products of this process are further utilized 1)y tlie snrviving ova.

Tlcc 1)cariity of se1wlir.e di?iz inat iot~ ~ p o n ora protluctioia i ic

tlc t) reinniiiiirg o u a ~ y af tcr scwz ispa?yiiig

Kcsults o1)taiiietl in this study furnish an aiiatomical hasis for esplaiiiiiig the cfYcct of scmispayiiig 011 tlic i i n m l ~ ~ r of

"A tlic*t:itecl stntemeut t o t h c s nr i te r . E'or f u i tlirr t1et:iils see p:ip(>rs listctl in 11 tei :it u i e cited.

SELECTIVE RLIMINATION OF OVA 457

ova produced by an animal. It has been shown that semi- spaying only slightly reduces the total number of ova pro- duced by an animal (Arai, '21; Allen, ' 2 3 ) . The remaining ovary nearly doubles its production to compensate, thus maintaining the number of ova typical for the species. This suggests, 1) that a fairly definite and probably limited amount of nutriment (in the sense of material from the blood stream or tissue fluids utilized by the ova) is available for repro- duction; 2 ) that this is sufficient for the maturation of only a limited number of ova, and, 3 ) that when one ovary is re- moved, the utilization of its share of reproductive nutriment by the remaining ovary lessens the severity of selective elimination and allows the maturation of nearly twice the number of ova which would normally have been produced in this ovary.

T h e application of this hypothesis t o conditions irz the ovaries of swine

Applying this hypothesis to the late growth of follicles in the ovaries of swine, shall we say that, a ) six to seven fol- licles (one ovary) at preovulation size are the metabolic eyiiivaleiit (as regards necessary nutritive material) of forty- five follicles of class I ; b) that with an increase in the size of the class I follicle Comes an increased demand for nutri- ment resulting in a struggle for the survival of the fittest? This would be merely the application to ovarian physiology of a universally accepted biological principle. It would ex- tend to the ripening ova the priiiciple of 'growth competition hetween organs,' recently developed by Stockard ( '21).

ConzparisotL of the arcrage ntimber o f definite ova prodlicrd with t J i P arcrage nunrbcr o f p igs p e r l i t ter

If tlic average iiumber of iiewly developed corpora Intea per ovary be doubled to account for the total productioii of both ovaries of an animal, ail average production of thirteen definitive ova per animal is obtained. Estimates of average

litttlr size in swiiie uiiselectcltl as to breed (S~irface, '08-'09) werc from SCVCII to nine pigs per litter. F~lliiiger ( ?I7 cyiiotetl from Siirfnce), in ii similar study of a spccial Ihmisli I)rectl iiotccl f o r i ts fecimtlity, computes a71 avcragc of 11 .,5 1)igs pcir littei.. Sincc the ovaries nsed in our work arc. from iiiisc~lcc~tetl sto('li, these estimates of the numher of dc4iiitive oi-a slioultl pro1)ably he compared with Siirface's results. Tliis i\-oultl mwi i a prenatal mortality of 30 to #i per cent. 1 liis cwmparisoii stresses tlie loss tlirougli iiitra-nteriiie mortality receiitly described and so fully discussed 11y Rohiii- soil ( 'X), Loiig aiid Kvaiis ( ' Z ) , I ih~uis aiitl I3isliop ( W ) ,

r ,

;111(1 C'ol'ller ( 723).

' o 111 ] )a i , i s o 1 1 of 11 t . ( , o r I I 1 at i o 1 1 9)) o v f a1 i t y u' i f 11 11 i i a t a 1 111 0 r f a1 i f y

'l'lie niitliors cited a h v e stress the poiiit that a high ratc ()f 1 jreiia t a1 mort ali t7"' oeciirs wi tli out the in t erveii tion of 1)atliological coiiclitions. From a review of this data a iiormal or pliysiologic.al elimiiiatioii of more tliaii 30 per cent seems ;I coiiscr\-ativc) figure f o r littcr-l)cariiig mammals. But tliis ixtc is figured upoii the number of clefiiiitive ova a s 100 per wilt, t l i e timc iiiterval rclpresciitiiig the wliole gestation lwrioti. Our data s11ow that the definitive ova prodiicetl in swiiie arc olily 1<5 per eciit of tlie arerage numbcr appearing as visilde follicles at about the midpoint of the ocistrous cycle, tlic time iiiterval I)ciiig not more tliaii t y o ivccks. It is oln-ious, tlicrc~fore, tllilt preiiatal mortality slionlcl lw coii- sitlcwd cliiefly" as a continuation of the process of selective c~limiiintion n-liicli operates to cveii a greater c>xteiit (liiriiig the late iiitrn-ovarian life of the ovum.

The work of Ev:iiis a i d Bishop ( '22, '23) on the reprothlc- tive vitamine, lias stressed the I m r i n g of iiutritioiial factors upon prenatal moi-tality. Kohiiisoii anel Corner liave stressed

I" Ikf i i i td lierc as e1iiiiiii:itioii occurring iii the period 1wtwet.n OJ u1:ction and Imrturitioii ; therefore i ~ i c l u d ~ i i g unfer t i l i~ec l ox:] : I S well :IS eliiii~ii:ltu1 zygotes.

The f:ictor nddcd by t l i c a spt'riii :ct f c ~ t i l i z : ~ t i o ~ l , of ~ O U I S C , , iiitrotl1lcc.s :I c.om- plimtion.

possible letlial or snbletlial factors in tlie zygotes. Un- doubtedly, both are important factors. From the extent of preovulatioii mortality, however, it docs not seem logical t o place so much importaiice upon defective gametic constitu- tion as a cause of selective elimination of ova. Of course, any sucli process falls more heavily upon tlie weak ; but slioultl we consider 85 per cent of class 1 follicles faulty in constitu- t ion? It seems to the writer that tlic stress iiaturallg falls upon a limited amount of nutritive material available for tlie reproductive activity of an animal, including both tlie piw(1uc- tioii of definitive ova and tlic birth of young. S tockad re- cogiiizes this principle, but considers that an acltlitioiial fa(*- tor (noii-specific) is operating “. . . . each growing hud (also true of embryonic organs) exerts a depressiiig infiiieiice oii tlie growth of all other buds in the ineliviclual plant.” Tt is possible tliat such a factor is operating in tlie ovar~‘ .

TIio beariiig of these results tipoia / l i p secretioii of t h e l i o i . i i i o l ? p

cazisiug growth cliaiigcs i i r tlic griiital tract

Several authors have pointed out a parallelism lietivecii the clevelopmeiit of tlie corpora lutea and certain changes in tlie genital tract, inferring that a hormone from the corpora lutea may be the cause of these changes. Our counts, showing N larger number of small arid medium-sized follicles than usually supposed to be present in tlic ovaries a t a d just before the midpoint of tlie cycle, provide an aiiatomical hasis wliich stresses the importance of the follicular 1iormoiie12 as the causative factor in the growth in the genital tract of ihv iioii-pregnant sow. This evidciicc removes the necessity for postulating the protluetion of a growth hormorlc h~ the corpora 1utcal3 in the absence of pregnancy.

We 11avr~ dcmonstratrtl tlic preseiicc of such :I hormone b y injections of follicular coiiteiits of hog oV:irieS illto s p y e d rats : i n t i mice (Allen and Doisy, ’ 2 3 ) .

I’ In more t1i:rn t w n t y s e p w t c prrp:irations we 1i:ive been unable to extract a growtli-produciiig llornioiie f rom corpora luten froin swine.

460 EDGAR ALLEX, WM. E. IiOUNTZ, EYBON F. FRANCIS

SUMMARY

1. A rigorous process of selective elimination is operating eveii in the late stages of the growth of ova in sexually mature ovaries of swine.

2. The period considered includes the teii to fourteen days preceding ovulation or the later one-half or two-thirds of the orstrous cycle ; therefore, merely a part of the intra-ovarian life span of the ovum.

3 . a t the beginning of this period there is an average of fortyfive visible follicles less Ihaii 3 mm. in diameter in tlie ovary. Only 6.5 follicles (15 per cent) successfully attaiii maturation arid are ovulated. Therefore, 85 per cciit of these small follicles are eliminated. 4. These data are compared with conditions in the ovaries

of tlie rat and mouse from the viewpoint of periodic oro- genesis (luring sexual maturity. 5. A limited amount of nutriment (material as utilized lyi

the ova) available for reproduction is suggested as a caiise of this rigorous selection in the production of the definitive ova. The 6.5 follicles a t preovulatioii size are considerecl the metabolic equivalent of forty-five class I follicles.

6. Tlie bearing of these results upoii the hypertropliy of tlie ovary remaining after semispaying is discussed. h less rigorous selection of ova in the remaining ovary due to double tlie amount of nutriment available for reprotlnction is sug- gested as the cause.

7. A comparison is made between preovulatioii aiitl prenatal moi-tality. If prenatal tleatli be considered chiefly a cow tiiiuatioii of this process of selective elimiiiatioii of ova, c?

similar explaiiation may he atlvaiiced as n iiatural physio- logical cause of iiitra-uterine mortality.

8. The large iinmher of small or medium sized visible folli- clcs iii the ovary at tlie mitlpoiiit of the cycle furiiislies iiii m a - tomical lmsis for tlie secretion of the g ro~ t l i -p1 .odu~ i i i~ hor- moiie whicli we l i a ~ e extracted from tlie folliculur conteiits a s t Iicl priiicipal causative ageiit in aiiabolic oestrous phenomena iti swiiie. There is no loiiger a need to postulate tlie produc- tiotr of sucli a liormoiicb by tlie c+orpora lutea in this species.

38 40 39 40 " 6 45

60 32 34

- - i l . )

60 70 90 70 .i 2 3" 30 (i 0

40 :% 0

38 1 7 4 3 G9 4 0 4!l :3 .i 7 1 3.5 5 0

2.5 80 4 5 70 3 8 6 5 3 .i 3 0 "4 80

24 60 52

70 7 It 35 45 30 50 48 30 33 45 40 60 2.5 :)r)

35 35

-- .);I

- -

70 43 35 4 -5 57 34 30 43 30 41

35 7 .T 37 4 0 4 .i

40

4 5 50

-- *>a

-- ;);I

2 1 78 "9 4 .i 39 30 5 2 40 42 40

70 3 0 44 30 35 28 57 36 72 60

'I The ~ i u m l ~ e r s of follicles of class I rrpreseiit in some cases actu:il counts of risil,le follirlrs; in other caws, niinini:il :ipprnririi:itioiis.

23 "0

8 14 19 "1 1 .j 2 (i 14 17

"7 1 7 14 14 17 1 7 1" 16

11 (i

3 4 11 11 1 .5 "3 12 1" 1 .i

20

)

30 1 G

9 "3 "3 16 1 6 16 "6 18 14 14 14 13

9 2 0 11 18 13

7 16 1 .? 18 14

7 13 2 3 l f i 16 13

13 18 1" "0 1 9 1 .? 11 1 9 1 9

9 10 I (i 4" "4 14 14 1 0 1 .? 24 20 "7 3 9

9 1 2

8 I .i

D "1 1 3 2 4

1 4 11 18 2 3 1 (i 9

1 !) 2 0 1" 1" 1 3 7 .5 1 !I 1 2 9

1 6 11 1 7 3 .5 13 18

9 1 .? 2" 2 1 14 18 I 9 24

7

"I 13 l!) 11 I.? 1 3 1 a 14 11

8 10 1 7 16 1 7 19 1"

14 24 14 27 1 .i 1" 9

I 6 12 "0 11 14 "0

)

1 (i 1 3 "3

I) 14 16 3" 1 4 19 1 -7 18 18 14 2 .i 31 1 .? 10 8

"0 "1 30 10 11 "0 11 "1 1 .5 24

6 1 7

11 30 1 8

8 14 14

9 "0 1.5 19 l f i 11 18 18 1 2

1) 18 10 1.5 14 11 10 8

26 18 1 2 "9 18 1 .? 13

1 .i 19 14 1" 2" 1 .i

11 1 .i 2"

13 16 18 1 .i 17 1 3 36 18 1 (i 8

18 3 .i 14 1.5 1 6 14 I .i 16 18 1 6

1

2 .i I d 9

17 18 18 13

20 19

9

30 1 ti 16 14 26 10 11 1 9

8 8

1 4 1 7 1 7 1 .? 9 6 11 10

8

)

0 '> -_

14 1 2 13

8 20 1" 19 30 15 19 14 t'G 16

8 9 8 I)

10 8

32 "0

9 10 I 7 "0 2"

17 2.5 " 3 11

-

10

4

)

ti

8 7

1 3 4

10

11 7 1

7

X

:i

7 ( I

)

10

4

7 7

7 )

11 8

(i

ti

>

11

ti

ti

11 ti

7 7 7

~

7 7

14 ti ti

4

t i

I;

4 (i

10 1 2

7

10 7 !I

!)

8

10

)

8

7 7 4

11 1

)

7 8

7 1

ti

X

ti

X

7

~~

li 7 !I

ti

7 7

10

1" 3

ti

8 10

8 t i

ti

X

)

7 ti

)

)

)

7 t i

9

>

8

>

8

8

7

7 li

1" 7

~

ti

10 8 4

7

4

6

9

>

1" 8

8

3 8

(i

1 0 10

8

ti

ti

7 4

>

ti

7 10 4

I)

)

)

10 1 0

)

)

7 )

~

7 7 8

7 li 7 I)

ti 9 )

ti )

1" 8

11 4

10

11 11 9 ti

12

ti

7

4 8

(i

I)

10 10 8

9 10

6 4

-

7 7 (i

ti 7 ti 9

I)

I)

"

7 9 8 I

4

4

ti 1

8

ti

ti

10 !)

ti I)

11 7 )

)

(i

)

li 7

11 4

~

7 4

>

8

3

8

10

4

8

8

7 1

3

3 3

1 .i 5 (i

6

ti >

ti

D

7 11

8

8

1

4

7 8

3 4 3 8

SELECTIVE ELIMINATION O F OVA 462

T I R I J E 8

Inridenre of follzcles of class IF' (larger than 9 nini. in tlwmeter) in 100 ovaries

8 G 3 > ) G 8 7 9 5 4 9 10 6 7 7 6 7 5 7

) 7 11 7 ) ) 5

7 5 7 9 5 7 G 1 2 7

) ) 8 1 G 2 7 8 G 7

3 7 4 2 2 7 4 ) 7 ) 10 4 4 3 4 3 4 G G 7 7 G 3 3 ) 6 8 4 5 4 3 6 4 5 4 G G 3 3 G 1 G 4 4

10 7 6 3 8 3 4 3 G 8

TABLE 9

Tlw incirltnce of reccnt c o r ~ ~ o r a lutea in 100 ovarics f rom swine chosen a t rnntionL to tlrtrrmine the average number of definitive ova produced

12 G 4 8 9 10 7 10 .i 9 4 7 ) 10 1

4 1 9 G G G G 5 )

7 G 5 7 8 1

) 7 8 10 8 7 8 7 8 8 G 5 G 8 -5 G G 10 10 )

G G 3 7 8 9 7 G 4 )

7 10 8 7 4 3 4 8 9 G 7 G 9 3 .i J G 7 7 10

4 6 8 4 6 5 4 7 G 9 4 G 4

J 6 4 4 G G

n

L I T E R A T C R E C I T E D

AKAI, IIAYATO 1920 On the postnatal developnieiit of the ovary (albino ra t ) , with especial reference to the nuniber of ova. Am. Jour . Anat., vol. 27. 1921 On the cause of the hypertrophy of the surviving ovary a f te r semispying (albino r a t ) and the number of ova in it. Am. Jour. Anat., vol. 28, no. 1.

ALLEN, EDGAR 1922 The oestrous cycle in the mouse. Am. Jour . Anat.. vol. 30, no. 3. 1923 Orogenesis during sexual maturity. Am. Jour . Anat., vol. 31, no. -1.

ALLEN, ,EDOAK, A N D DOISY, EDWARD A. 1923 An ovarian hormone: :L prelimi- n:iry report on its localization, extraction, and partial purification, :rnd action in test animals.

ALLEN, EDGAR, DOISY, EDWARD A., F R a N C I S , B. F., GIBSON, H. V., ROBERTSON, L. L., COLGATE, C. E., KOUNTZ, W. B., AND JOHNSTON, C. G. 1924 The hormone of the ovarian follicle; its action in test animals. (AIJ- strnct.) Proc. I'hys. Soc., Am. Jour . Pliys., vol. 19.

J. A. M. A., vol. 81, p. 819.

HALFOUR, E’. 51. 1878 011 the s t ructure :tiid tlerclopnrent of tlie vertc!l)r:ite

~ ~ u R R o n . s , h1. T. 1017 Studies on the inet:ilio1isni of cells ill VitI’ci. Johns 1~1011- or:iry.

kins Hosliit:il licports, rol. 18. 1917 Studies on the growth of rells. Jour . Urol., vol. 1, no. 1.

CORNER, GEO. W. 1 9 2 1 Tlic or:iri:in cycle of swine. Science, N.S., vol. ,53, no. 1374, p. 420. 1!)21 Cyclic c h n g e s in tlic owritxs and uterus of the sow, and their relation to the ni~~c11:inisni of inip1ant:Ltinii. (I:nnegio Pul). , ( h i t .

I!E3 The prolileni of enil)ryonic p:itliology in m;immals, with oliser- ra t ions on iiitr:t-uterine mort:ility in tlic pig. Am. Jour . Anat., vol. 31, no. 5.

Isisis of their genetic potenti:iliiies. ELLIXGER, TAGE 1921 (Quoted from Surface.) Tlic influence of age on f r r -

tility iii swincs. EVASS, 11. M., AND BisIiop, I<. S. On the rt~1:itions between fer t i l i ty :ind

nutrition. 1. The orn1:itioii rhytlrni in tlie rat on a stantland nutr i - tioii:tl reginic~. Jour. Met. 11cs., ro l . 1, p. 319. 11. The oru1:ttion rliythin in the r a t on in:itlequ:ite 1iutrition:il regimes. lbid. , 11. 3 3 5 . 111. The norrn:il rqiroductivt. 1wrforni:ince of tlie rat. Jour. Met. lies., vol. 3, no. 2 . IV. Tlie prodnction of sterility with nutritional rcgiines :Ldcquatr for growtli :itid i t s cwre with otlier foodstubs. Ibid., 11. 233.

KINGSBI:KY, U . 3’. 191 3 The niorpliogeuesis of the m:imin:ilian awry ; Felis tlonicstic:i.

L O N G , J. A,, AND EVANS, 11. 13. 1922 The oestrous cycle in the ra t :in11 its assocaiatcd plienorrien:~.

~ ~ ( ’ R E N z I E , FKED F. ‘I’lie corre1:ction of esteri1:rl signs and v:igiii:il c1i:cngeli mitli t l i r or:iri:in cyr l r in swine. Alistwct, l’roc. A. A. A,, Air:it. Iiec., vol. 27, no. 4 .

I ’APANIVOLAOU, G. N., AND STO(~KAKD, (7. li. l!Ul Morphology of cystic growths in tlie awry :ind uterus of tlic giiine:i-pig. I’ror. Roc. Esp. Biol. :ind Xled., rol. 19, 1). 401.

I’EAKL, RAYMOND 1!)17 The e x ~ ~ c r i ~ ~ i e ~ i t : i l irioclific:ition ~f germ cells. Jour.

I’EARI,, I?AYMONI), .4ND S c m o r P E , WX. F. l!El Studies on the physiology of rqiroiluctiou in tlie t l o n i c ~ s t i v fowl. XVIII. Further ol)serv:itions 011

11ie : in:~to~i i ir : i I Ixisis of fecundity. Tlic forni:ition, rupture :l i lt1 closurc of ov:~ri:iii follicles

in f<.rrets :ind ferret-l)ole-cxt liyl)ritls, :ind some :issoci:ctetl p l~eno~n(~n:~. Roy. Soc., Edinl)urgh, vol. 52, pt. 2 , 110. 13. ]!El 1’r~~n:it:iI t1e:itli. b~‘:t l inl~n~gli 31ctl. Jour., ro1. 26, March.

Qu:irt. J o u r . Rlicro. Sci., ro l . 18.

Emb., no. 64.

I)ANFOItTII, ( y . 11. 1919 .EVitlC‘llC(l tll:lt g(’l’lll CC’llS :Ire SLl!),jWt t0 SeleCtiOlI 011 tile J o u r . Exp. Zoijl., vol. 28, no. 3.

I’roc. Nat . A w t l . Sci., rol. 7, 110. 5, p. 134. 1922

Am. Joiir. An:ct., rol. 1;.

Mein. Unir . Calif., ro l . G . 1924

Eq,. ZOOI., rol. 2 2 .

J o u r . Exp. Zoijl., vol: 34, no. 1. ROHIXSON, AKTIIUR 1918

Ssvnelc, F. P. Ch:ciiq~s in the f:illoL)i:in tulw during the ovul:itioii c y c l ( ~

S T O ( K A R D , C . 1%. I!Vl I ~ c ~ r c l o ~ ~ i n ~ ~ ~ i t : ~ I 1’:itv :rnd struc.tur:il c~qiression : xn cs- ]~~rinient : i l stutly of twitis, tloul)le inoiistcrs :Inti single deforrnities, :11i(1 tlie int(~r:iction :iiiiong c~r i i l~r~i in io 0rg:ins during tllrir oi.igili :ni(l

tlcrt~lolinicwt. 4111. Jonr. An:lt., Val. 28, no. 2 .

1923 :I1111 c : I ~ I ~ ~IY~’~II : I I I ( ’~ . J. H. IIOSli. I~uII., VIJl. 34, 11. 121.

SELECTIVE RLIMINATION OF OVA 465

STOCKARD, C. R., AND I’APANICO~~AOU, (1. N. Further studies on the modi- fication of germ cells iii m:iiiiainls. Jour. Exp. Zoiil., vol. 2G.

STBUVPI, J. 1!)11 (Quoted from Corwr.) Die Pcriodcw der Brunst bei Itiudern, Scliwcineu uiitl Pferden. Fullling’s laiuclwirtscl~nft. Ztg. Jahrgring, Bd. 60, S. 832-838.

1918

SUEFACE, F. M. 1908-1POO Fecundity of swine. Biometrikn, vol. G, p. 443.

PLATE 1

EXPLANATION OF FIGURES

Oviiries of swine chosen as representative of diff e r m t stages of follicular development ancl grouped in tlie following classes for the purpose of diff ereiitial counts:

Class I, smaller than 3 mm. in diameter. Class 11; 3 to 6 mm. in diameter. Class 111; 6 to 9 mm. in diameter. Class IV; larger than 9 mm. in diameter. Clriss V ; ruptured follicles represented by large red corpora lutea. 1 ( I ) ; 68 small follicles, of which two a re larger than class I; no corpora

lutaa are present in this ovary. 2 ( I ) ; 73 small follicles, all of approximately the same size (the follicles

of this class already contain liquor folliculi); one set of more tliaii 7 large red corpora lutea (listed as class V).

3 (11) ; 18 follicles; more than 6 large red corpora. 4 (11); 14 follicles; a set of medium-sized corpora. 5 (111); 11 follicles of an average d h n e t e r of 7.5 mm.; 5 or 6 smaller ones,

possibly in the process of elimination; there is a set of small corpora in this ovary.

6 (111); 6 follicles, but no corpora lutea. 7 8 (111); 14 follicles of approximately the same size; 3 white corpora, one

9 10 ( IV) ; 9 large follicles; no corpora. 11 ( I V ) ; 12 large follicles; a set of small white corpora. 12 (IV) ; 4 large follicles; a set of small white corpora. 13 ( IV) ; 6 follicles and 2 sets of white corpora. 14 (V) ; 10 large follicles and a set of rery small corpora. Blood vessels

a re prominent in tlie clear transparent follicle walls. 15 (V) ; 5 large follicles, possibly in a n early stage of Cystic degeneration,

although apparently normal ova may be reeorcred by aspiration of the contents.

(111); 7 follicles and 7 white corpora.

nearly hidden by follicles in t!ie center of the pliotograpli. ( I V ) ; 10 large follicles; a set of small white corpora.

4GG

467