iv THE SEX CHROMOSOMES 107 



the linear or diffuse stages characteristic of syndesis and other phases 

 of the meiotic prophase. 



Since where Y is absent it is' obvious that the X chromosome, having 

 no mate, cannot go through the process of syndesis like the other chromo- 

 somes, the natural conclusion is that the visible difference between the 

 behaviour of the ordinary and sex chromosomes is the expression of the 

 fact that the latter is not taking part in syndesis. Even when a Y 

 chromosome is present, both it and the X chromosome often remain 



T 



Protenor Syromasbes Ascaris 



AnascL lumbricoides 



Y 



X 



t 



Nezrara, Euschistus Nezara 



viridula, Coenurus hilaris Thyanta 



Y 



X 



M 9 * 



Rocconola Prionidus Gelaslocoris Acholla 

 FitchicL Sinea, mullispinosa, 



FIG. 51. 



Diagram of the relations of the sex chromosomes in various animals. (After Wilson, A.m.A., 1911.) 



compact throughout the meiotic prophase, and no evidence of syndesis 

 between them can be found. The absence of conjugation is not surprising 

 in view of the physiological differentiation between the X and Y chromo- 

 somes which must underlie the frequent difference between them in 

 regard to size and composition. Moreover, as we shall see later, the facts 

 of sex-linked inheritance (p. 179) lead to the conclusion that the Y 

 chromosome is inert. 



If the absence of syndesis be the explanation of the compactness of 

 the X and Y chromosomes during the zygotene stage in the male, it 



