STUDIES ON SEX-DETERMINATION IN AMPHIBIANS. 3! 



III. THE INFLUENCE OF THE RIPENESS OF THE EGG AT THE 

 TIME OF FERTILIZATION ON SEX-DETERMINATION IN BUFO. 



For several years past Hertwig ('05 -'07) has been investigat- 

 ing the problem of sex-determination, principally in various species 

 of Rana, and he has already published three contributions to the 

 literature on this subject. In his first paper, which appeared in 

 1905, Hertwig ('05) suggests that the sexual character of an egg 

 changes during the course of its development in the ovary. In the 

 early stages of its ripening the egg tends to produce a male ; in the 

 middle phase of its ripening its tendency is towards the production 

 of a female ; and in later phases, when the egg is overripe, it again 

 shows a tendency to develop into a male. This change in the sexual 

 character of the egg Hertwig ascribes in his second paper ('06) to 

 "einem verschiedenen Wechselverhaltnis ihrer Hauptbestandteile, 

 der Kernsubstanz und des Protoplasma. . . . Eier, welche rela- 

 tiv armer an Kernsubstanz sind, Weibchen liefern, chromatin- 

 reichere dagegen Mannchen." In his latest communication Hert- 

 wig ('07) lays little emphasis on the second part of his theory, 

 but he seems to consider that the results of his experiments war- 

 rant the conclusion that the ripeness of an egg at the moment of 

 its fertilization determines its sex. In explaining the results of 

 a certain set of experiments in which the eggs of a female from 

 one locality were fertilized with sperm from a male taken from a 

 different locality, Hertwig further modifies his theory by the sug- 

 gestion that in some cases the spermatozoon exerts a definite in- 

 fluence on sex-determination, although, as a rule, it is probable 

 that " die Eier zur Zeit der Befruchtung sexuell in so hohem Grad 

 determiniert sind, das der relativ geringe Einfluss des Samens gar 

 nicht zur Geltung kommen wiirde." 



Morgan ('08) has recently given a brief review of this work of 

 Hertwig's on sex-determination and also a criticism of the results. 

 He states that " Hertwig's attempt to connect his view with the 

 ratio of nucleus to cell-plasm of the egg at different periods of 

 its maturation can hardly be looked upon favorably, since in the 

 frog's egg the nucleus as such has already disappeared when the 

 egg leaves the ovary. The chromosomes are thereafter arranged 

 on the equatorial plate of the first polar spindle. It is, however, 



