REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1115 



the stigma, is the place where the follicle 

 ruptures. On microscopic examination, small 

 blood vessels are found to cross the stigma 

 (Xalbandov and James, 1949 ». In the theca 

 in tern (I of the rook Marshall and Coombs 

 (1957) found large glandular cells whose 

 cytojilasm contains lipid droplets, similar 

 to those in the Leydig cells of the male. The 

 cells are considered by Marshall and 

 Coombs to be the source of estrogen. In ad- 

 dition to the thecal gland cells, Marshall 

 and Coombs distinguished exfollicular gland 

 cells which may arise from fibroblasts that 

 migrate from the theca into the lumen of 

 atretic follicles. After collapse of such fol- 

 licles till' cells are freed into the stroma. 

 Although they resemble the Leydig cells 

 histologically these cells are not homologous 

 with them, because they arise from follicles 

 and not from stromal tissue. The female 

 Leydig cells, which may be considered ho- 

 mologous with the male Leydig cells, arise 

 from connective tissue cells and may be the 

 source of androgen in the female. After rup- 

 ture of the mature follicle the thecal gland 

 cells disappear and the empty follicles are 

 invaded by erythrocytes, lymphocytes, and 

 a very large number of fibroblasts. In birds 

 tliere is no structure that may be regarded 

 as homologous with the mammalian corpus 

 luteum. The references to the ruptured fol- 

 licles as an avian corpus luteum by Pearl 

 and Boring (1918), Novak and Duschak 

 (1923), and Bradley (1950j are erroneous. 



3. Function of the Ovary 



The avian ovary produces gametes and 

 hormones which play an integral role in the 

 production of the egg. 



Gametogenesis. Gametogenesis starts in 

 the embryo, so primary oocytes with chro- 

 mosomes in the bivalent state (Hutt, 1949) 

 at the time of hatching are present in the 

 ovary. During the long interval between the 

 time of hatching and about 4 to 5 hours 

 before ovulation, little activity takes place 

 in the nucleus. In sharp contrast, large 

 amounts of yolk are deposited in the follicle 

 in the 8 to 9 days before ovulation. 



About 24 hours before ovulation, the 

 breakdown of the germinal vesicle starts 

 (Olsen, 1942), but nuclear changes are not 

 vet noticeal)le. The reduction division is 



lar layer of stalk 



Fig. 18.10. Hi.stulugy of ovanan folLcle of the 

 chicken according to Nalbandov and James (1949). 

 (From A. V. Nalbandov and M. F. James, Am. J. 

 Anat., 85, 347-378, 1949.) 



comjileted about 2 hours before ovulation in 

 both the fowl and the turkey (Olsen and 

 Fraps, 1944, 1950). Thus the sex of the fu- 

 ture embryo is determined at ovulation and 

 not at fertilization, as it is in mammals. 

 Investigations to determine the primary sex 

 ratio on fowl, possible only in hatches with 

 100 per cent fertility and no embryonic 

 death before sex difTerentiation, have shown 

 it to be unity ( Hays, 1945 ; Landauer, 1957) . 

 The nuclear changes in the primary oocyte 

 leading to extrusion of the first polar body 

 and formation of the secondary oocyte are 

 apparently under the control of LH, be- 

 cause LH injections produce such changes 

 prematurely (Olsen and Fraps, 1950). The 

 secondary oocyte extrudes the second polar 

 body after sperm penetration of the vitelline 

 membrane (Olsen, 1942; Olsen and Fraps, 

 1944). 



During gametogenesis aberrations can oc- 

 cur which lead to parthenogenetic embryos. 

 In certain strains of turkeys the incidence 

 of parthenogenesis may be as high as 100 

 per cent ( Poole and Olsen, 1958 ) . The occur- 

 rence of parthenogenesis in birds may have 

 been discovered as early as 1872, as was 

 pointed out by Fraps (1955). Partheno- 

 genesis occurs in turkeys (Olsen and Mars- 

 den, 1953, 1954, 1956; Olsen, 1956) and 

 chickens (Olsen, 1956; Poole and Olsen, 

 1958). Only a small fraction of the eggs 

 showing parthenogenetic development de- 

 ^'eloped into normal embryos and only a few 



