1112 



SUBMAMMALIAN VERTEBRATES 



embryo was female if less than 41 per cent 

 of the total number of primordial germ cells 

 was present in the right gonad, whereas it 

 was male if more than 45 per cent were pres- 

 ent in the right gonad. Simon (1960) on the 

 basis of an extensive study concluded that 

 van Limborgh's conclusions were erroneous 

 and that the ratio of primordial germ cells 

 in left and right gonad of the 25 to 32-so- 

 mite stage chick embryo cannot be used to 

 determine the sex of the embryo. As both 

 authors base their conclusions on statistical 

 considerations of normal and abnormal sex 

 ratios further work using cytologic tech- 

 niques needs to be performed. Kosin and 

 Ishizaki (1959) established that the pres- 

 ence or absence of sex chromatin in the 

 nucleus permits sex identification of chicks. 

 The sex chromatin is found in birds in the 

 cells of females as it is in mammals, in spite 

 of the fact that in birds the female is the 

 heterogametic sex. Ohno, Kaplan and Kin- 

 osita (I960) have proposed that the sex 

 chromatin of chickens represents a single Z 

 chromosome in positive hcteropycnosis. 



Stanley and Witschi (1940) compared the 

 asymmetric gonads of embryonic chicks 

 with the symmetric gonads in Accipiter 

 cooperii, Buteo amiansis horealis, and Cir- 

 cus cyaneus hudsonius. They found that the 

 distribution of primordial germ cells be- 

 tween the left and right gonads in these 

 latter species was still essentially sym- 

 metric at a stage of development when 

 asymmetry already had developed in the 

 chick. They offered the explanation that, in 

 the chick, primordial germ cells migrate 

 from the right to left, thus disturl)ing the 

 initial symmetric arrangement. Van Lim- 

 borgh (1957) tested this hypothesis by 

 dividing the duck embryo medially and 

 thus destroying all vascular and other con- 

 nections between the left and right side. 

 In the surviving embryos the asymmetry 

 was not different from untreated controls. 

 On the basis of other experiments van 

 Limborgh came to the conclusion that the 

 asymmetric distribution of primordial germ 

 cells could not be explained by the fact 

 ^■i-'at the embryo lies with its left side to- 

 wards the yolk so that the left gonad may 

 be better A^ascularized (an explanation pro- 

 posed by Dantschakoff and Guelin-Sche- 



drina (1933) nor could it be explained by 

 the secretion of hormones by the gonads 

 which inhibit or stimulate the asymmetric 

 migration of the primordial germ cells. 



The riddle of the asymmetrical distribu- 

 tion of the primordial germ cells still re- 

 mains unsolved and no real explanation can 

 be given for the difference between the 

 hawks and other birds. The right ovary of 

 the hawks is apparently functional, for no 

 histologic differences between it and the 

 left ovary have been found (von Faber, 

 1958). Domm (1939) states that yolks can 

 be ovulated from these right ovaries; he 

 postulated that such yolks might be trans- 

 ported through the left oviduct, for the 

 right oviduct is either missing or vestigial 

 (Stanley and Witschi, 1940; von Faber, 

 1958). Domm based his hypothesis on the 

 absence of yolk material in the body cavity. 

 It has since been shown that yolk can be ab- 

 sorl)ed from the body cavity in less than 24 

 hours (Sturkie, 1955a). An additional rea- 

 son for believing that Domm's hypothesis 

 is probably incorrect is that the dorsal mes- 

 entery makes it impossible for a yolk to 

 move from the right to the left side of the 

 body cavity. This anatomic arrangement 

 occurs in chickens and it probably is the 

 same in hawks. It is rather difficult to un- 

 derstand the evolutionary significance of the 

 presence of two ovaries with only one ovi- 

 duct and the occurrence of this arrange- 

 ment in only one order. 



The postembryonic development of the 

 fowl's right gonad after sinistral ovariectomy 

 has been investigated in different labora- 

 tories. Domm (1939) reviewed the litera- 

 ture which had been published to that time. 

 Only the salient features pointed out by 

 Domm and new evidence obtained since 

 then will be presented here. 



Ovariectomy before 30 days of age results 

 in the development of the right gonad into 

 a testis or an ovotestis, either of which may 

 exhibit active spermatogenesis (Domm, 

 1939; Kornfeld and Nalbandov, 1954; 

 Kornfeld, 1957) sometimes even at an ear- 

 lier age than in cockerels of the same breed 

 (Taber, Claytor, Knight, Flowers, Gambrell 

 and Ayers, 1958). Later ovariectomy re- 

 sults in a greater incidence of ovotestes and 

 ovaries than does ovariectomy before 30 



