4i8 Experimental Zoology 



part — is used in the formation of the tail of the spermatozoon, 

 which may or may not enter the egg; but some of the cyto- 

 plasm forms at least the centrosome and is carried into the egg. 

 Owing to our inabihty to follow the history of this cytoplasm 

 in the egg, we have come to ignore its existence, while the nucleus 

 that can be easily traced has occupied exclusively the attention of 

 modern embryologists with rare exceptions. 



If we admit that the cytoplasm of the spermatozoon is not only 

 brought into the egg, but slowly increases in quantity there, can 

 we find any clew in such a condition that will help in the solu- 

 tion of the problem of sex determination? There are several 

 possibihties that should be considered. If we suppose that there 

 is predetermined male and female cytoplasm in the egg or 

 the spermatozoon, or in both, we encounter precisely the same 

 difficulties that have been met with by assuming that predeter- 

 mined elements of these two kinds exist in the nucleus. From 

 this point of view the two assumptions stand on the same footing. 

 If we assume that the determination of sex Hes in the cytoplasm, 

 not in the form of predetermined elements male and female, 

 but as one of the alternative conditions of differentiation of the 

 cytoplasm, we must still explain what factors — external or in- 

 ternal — determine whether the one or the other condition shall 

 dominate. Whether this is due at times to a relation between 

 the chromatin and the cytoplasm, or at other times to a relation 

 depending on the condition of nourishment of the cytoplasm, 

 etc., cannot be stated. However probable it might be made to 

 appear that the differentiation that appears in the cytoplasm 

 really originates there, and not in the nucleus (except in so far 

 as the latter induces important changes in the cytoplasm through 

 its assimilative changes), the fact remains that we cannot ex- 

 plain the mechanism in the cytoplasm through which the one or 

 the other condition comes to be the dominating one. Here, as in 

 the case of the nucleus, we are too ignorant at present of the real 

 chemical changes that take place to permit of more than purely 

 speculative views. 



Ziegler has recently proposed an hypothesis of sex determina- 



