THE MECHANISM OF SEX-DETERMINATION 55 

 THE EXPERIMENTAL EVIDENCE 



The experimental evidence, indicating that there is 

 an internal mechanism for sex determination, is derived 

 from two sources — from experimental embryology, and 

 from a study of the heredity of sex-Hnked characters. 



The evidence from embryology shows that the chro- 

 mosomes are the bearers of materials essential for the 

 production of characters. The evidence from hered- 

 ity shows that certain characters follow the sex 

 chromosomes. 



It has long been taught that the hereditary factors 

 are carried by the nucleus. The evidence for this was 

 found in fertilization. When the spermatozoon enters 

 the egg, it carries in, as a rule, only the head of the sper- 

 matozoon, which consists almost entirely of the nucleus 

 of the original cell from which it comes. Since the 

 male transmits his characters equally with the female, 

 it follows that the nucleus is the source of this 

 inheritance. 



The argument has not been regarded as entirely 

 conclusive, because the sperm may also bring in some 

 of the protoplasm of the original cell— at least that part 

 lying immediately around the nucleus. In addition a 

 small body lying at the base of the sperm head seems 

 also to be brought in by the male, and according to 

 some observers it becomes the center about which the 

 entire division system or karyokinetic spindle develops. 



The most convincing evidence that the chromosomes 

 are the most important elements in heredity is found in 

 some experimental work, especially that of Boveri, 

 Baltzer, and Herbst. Under certain circumstances in 



