IV 



THE SEX CHROMOSOMES 



107 



the linear or diffuse stages characteristic of syndesis and other pliases 

 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 hke 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 



Prole nor 

 AnascL 



it 



Syromastes 



As cans 

 lumhrtcoides 



y 



X 



I 



Nezrara. 

 virtdulcL 







Euschistus 

 Coenurus 







Nezara 

 KiloLris 







f 



ThyantcL 



Y 

 X 







RocconotcL 

 FttchicL 







V 



Prionidus 

 SinecL 







0. 



GelcLstocoris Acholla 



mix ll ispin osa 



Fig. 51. 



Diagram of the relations of the sex 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-hnked inheritance (p. 179) lead to the conchision 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 



