2. 



Since the time when this explanation of what is now 

 known to be an instance of the inheritance of sex-linked 

 characters was offered, the sciences of cytology and genetics 

 have expanded vastly and out of their development emerged 

 the current theory of sex-determination. By this theory all 

 the older hypotheses of sex-determination have been ren- 

 dered obsolete and, save for historical purposes, can be 

 disregarded. The framework of this theory consists of secure 

 knowledge concerning the constant and significant differ- 

 ences between male and female in respect of their sex- 

 chromosome and therefore of their genie constitution and, 

 secondly, concerning the observed facts relating to the 

 phenomenon of sex-linkage, which enables the observer to- 

 trace the transmission of sex-chromosomes from generation 

 to generation. The primary difference between the sexes is 

 now known to be a chromosomal and a genie difference. In 

 the bisexual and dioecious species the sex of the individual, 

 as a general rule, is now known to be decided at the moment 

 of fertilization by the interplay of the sex-chromosome 

 constitutions of the uniting gametes. This chromosomal,. 



