IV 



THE SEX CHROMOSOMES 



105 



consisting of two or more components. These are separate in the somatic 

 or premeiotic nuclei of both sexes, and in all nuclei of the female (with 

 the probable exception of Phylloxera caryaecaulis, see p. 117) ; in the 

 meiotic phase of the male, however, they commonly become associated 

 in a degree varying from merely a more or less close grouping (Acholla) 

 to an actual junction (Syromastes) (Figs. 48, 49, 50). The Y chromosome 

 is always simple, even in those species where the X is compound. The 



X" 

 Y 



-Y 



FIG. 49. 



H 



> 



Vi* 



The chromosomes of Syromastes (A, B) and Acholla multispinosa'^C-l). T (Wilson, B.B., 1909 a, and Payne, 

 Z?./?., 1910.) In A and B the X chromosomes are blackened. A, <$ diploid group, 20+ X%= 22 chromosomes ; B, 

 9 diploid group, 20 + 2X^ = 24 chromosomes ; C, 9 diploid group, 20+2X5 = 30 chromosomes. The three pairs 

 of small X elements are easily seen, the other two pairs are not at this stage distinguishable. D, Q" diploid 

 group, 20+X 5 +Y = 26 chromosomes; E, metaphase I., 6, polar view. The Y element is characteristically 

 in association with the two largest of the X elements. F, G, anaphase I., side view cut in two sections. The 

 section in F contains the Y and the two large X elements, and that in G the three small X elements. H, I, 

 anaphase II., showing the results of the segregating division. 



following examples illustrate the range of variation in the constitution 

 of the sex chromosome (Fig. 51) : 



(A) No Y chromosome present. 



1. X Chromosome single (Protenor). 



2. X ,, double (Syromastes). 



3. X ,, pentad (Ascaris lumbricoides). 



(B) Simple Y chromosome present. 



i. X Chromosome single (Lygaeus, Oncopeltus). 

 ,, double (Fitchia). 



2. X 



3. X 



4. X 



5. X 



triple (Prionidus). 

 quadruple (Gelastocoris). 

 pentad (Acholla multispinosa}. 



