212 Basis of Sex Determination 



I 



coming from one egg have the same chorion and can 

 thereby be diagnosed as such. They can be produced 

 as we have stated in Chapter V by a separation of the 

 first two cleavage cells of the egg, each one giving rise 

 to a full embryo. It harmonizes with all that has been 

 said above that the sex of two such individuals must 

 be the same since they have the same number of X 

 chromosomes, the latter being determined in the human 

 race by the nature of the spermatozoon which enters 

 the egg. 



4. While thus far all the facts agree with the 

 dominating influence of certain chromosomes upon 

 sex determination, one group of facts has not yet been 

 explained: namely, hermaphroditism. By hermaphro- 

 ditism is meant the existence of complete and separate 

 sets of female and male gonads in the same individual. 

 This condition exists regularly not only in definite 

 groups of animals, e. g., certain snails, leeches, tape- 

 worms, but also, as everybody knows, in flowering 

 plants. While in some forms both kinds of sex cells, 

 male and female, are formed and mature simultaneously, 

 as, e. g., in the Ascidian Ciona (see Chapter IV), in others 

 they are formed successively, very often the sperma- 

 tozoa appearing first (protandric hermaphroditism). 

 In the long tapeworm TcEfiia each ring has testes 

 and ovaries, but the young rings are only male while 

 in the older rings the testes disappear and the ovaries 

 are formed. The same ring is in succession male and 



