EFFECTS OF GONAD GRAFTS IN CHICKS 13 



a. Sex differentiation. R. Semen ('87) described the differ- 

 entiation of sex in the chick in the following words : 



Die geschlechtliche Differenzierung, die sich beim Huhnchen am 

 5., spatestens am 6. Tage erkennen lasst, macht sich zunachst dadurch 

 bemerklich, dass bei weibHchen Embryonen die rechte Keimdrtise in 

 auffallender Weise im Wachstmn zurtickbleibt. Auch beim Mannchen 

 entwickelt sich haufig der rechte Hode langsamer und bleibt oft zeitle- 

 bens kleiner. Aber die Differenz ist hier stets so unbedeutend, dass 

 man am 6. Tage immer ohne Miihe Hode und Eierstock an diesem 

 Merkmal unterscheiden kann. 



Later Swift ('15), on histological grounds, stated: 



When the chick embryo has reached the 156th hour of development 

 (6| days), the formation of cords of first proliferation ceases rather 

 abruptly and it is about this time that the sex of the individual can be 

 definitely determined. In the determination of sex there are three 

 criteria on which reliance can be placed. These are the relative size 

 of the two gonads, the germinal epithelium, and the number of pri- 

 mordial germ-cells in the germinal epithelium. ... an interesting 

 fact and one to which attention has not been previously called, is 

 the presence of more primordial germ-cells in the germinal epithelium, 

 of the left female gonad than in the epithelium of the male gonad. 



From these two quotations it is evident that the first signs of 

 sexual differentiation, both niicro- and macroscopic, appear 

 between the fifth and seventh days of incubation. The most 

 readily recognizable character is the difference in size between 

 the right and left gonads in the female. Besides this size dif- 

 ference, a difference in shape between male and female gonads is 

 also noticeable. In some cases the retrogression of the miillerian 

 ducts in the male had already begun on the sixth day. We may 

 therefore say in general that sexual differentiation appears 

 toward the end of the first week of incubation. 



h. Degeneration of the right ovary. Both ovaries develop 

 equally until the end of the first week of incubation, that is to 

 say, the development of the ovaries is symmetrical up to the 

 time when sexual differentiation appears. But from this time 

 on the development of the tw^o ovaries is asymmetrical. The 

 left ovary continues to increase in length and size, while the 

 right ovary undergoes a process of degeneration and finally 



