SPECIAL PROBLEMS OF MITOSIS 



41 



and X^. Thus in the Praying Mantis the chromo- 

 some sets of the two sexes are : 



Male 13 pairs of autosomes, X^ X^, Y 

 Female 13 „ „ X^, X\ X\ XK 



In this case it will be seen that the heterogametic sex 

 has an odd number of chromosomes, but a Y is 

 present. ^^ The two ends of the Y are probably 

 homologous to regions in the X^ and X^ respectively 

 (Fig. 86). 



y Y I ^ »v\\\\\\\\\\\\\\\\\\\^ X 



sSme^t Differential Segments 



^t^ ) Y 



S.A. 



Mechanism 



S.A. 



M 



XiXjY 



T^^/.. Differential Segments ^^9^ 

 Mechanism\(\ 



SA. 



X, 



S.A. 



X 



Y 



Fig. 8. — Diagrams of the sex-chromosomes (above) the male 

 rat, and (below) the male Praying Mantis. The homo- 

 logies of the various regions are indicated diagram- 

 matically. S.A. = spindle attachment. 



In the insect Perla marginata ^® there is a similar 

 situation but a Y is absent so that the two sets are : 



Male 10 pairs of autosomes, X^, X^ 



Female 10 „ „ X^, X^, X^, X2. 



In some cases it is the Y which is represented by 

 two separate chromosomes, there being only one X. 

 Thus in the dioecious Sorrel Dock, Rumex acetosa, 

 we have : 



