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 S5nidesis 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 



X 



T 



Protenor 

 AnascL 



it 



Syromasbes 



Ascaris 

 lumbricoides 



y 



X 



% 



Nezura, 

 viridalct 





 I 



Euschistus 

 Coenurus 







Nexara 

 hilar is 



1_ 

 f 



TnyantcL 







Y 

 X 



RocCOTlotCL 



Fitch io. 



0. 



V 



Prionidus 

 Sineo, 







Gelastocoris 







Acholla 

 mult Lspinosa 



Fig. 51. 



Diagram of the relations of the ses chromosomes in various animals. (After Wilson, A.m. A., 191 1.) 



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 underhe 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 



