50 HEREDITY AND SEX 



I shall pass now to a fourth condition that has only 

 recently come to light. It is best shown in some of the 

 nematode worms, for example, in the ascaris of the 

 horse. Here the sex chromosomes are generally at- 

 tached to other chromosomes. In this case, as shown 

 by the diagram (Fig. 27, A), there is in the male a single 

 X attached to one of the other chromosomes. At the 

 first spermatocyte division it does not divide (C), 

 but passes over bodily to one pole, so that two kinds 

 of cells are produced. At the second spermatocyte 

 division it divides, in the cell that contains it, so that 

 each daughter cell gets one X (D). Two classes of 

 sperm result, two with X {E), two without (E'). 



In the female there are two X's, each attached to a 

 chromosome (Fig. 28). After the polar bodies are 

 given off, one X only is left in each egg (C, D, E). Sex 

 is determined here in the same way as in the insects, 

 described above, for there are two classes of sperm and 

 but one class of eggs. 



The discovery of the sex chromosome and its rela- 

 tion to sex is due to several investigators. In 1891 

 Henking first described this body, and its unequal distri- 

 bution, but was uncertain even as to its relation to the 

 chromosomes. Paulmier (1899), Montgomery (1901), 

 Sinety (1901), gave a correct description of its behavior 

 in spermatogenesis. McClung (1902) confirmed these 

 discoveries, and suggested that the accessory, or odd 

 chromosome, as it was then called, had some relation 

 to sex, because of its unequal distribution in the 

 sperms. He inferred that the male should have one 

 more chromosome than the female, but he gave no evi- 

 dence in support of this suggestion, which as we have 



