258 WM. REES B. ROBERTSON 



Vs. It might also be imagined that only the ends of such split 

 compound chromosomes should conjugate and not the mid- 

 regions. That such presynapsis splitting is possible, we are 

 led to suppose from the reports of Dehorne ('11) and Schneider 

 ('10) in somatic mitoses and Brunelli ('10, '11) in last sperma- 

 togonial mitoses, who found that the telophase chromosomes 

 on their way to the poles show longitudinal splitting. Before 

 I knew of any of these works (thus eliminating the chance of 

 error by suggestion), I had found similar conditions in the ana- 

 phases of both somatic cells and the last spermatogonial divisions 

 in the Tettigidae. My conclusions from the Tettigidae are, 

 that these spermatogonial chromosomes on entering synapsis 

 are already split. This may be seen, in each of the chromo- 

 somes of cells where the full diploid number exists. Before 

 synapsis the split disappears. The twelve autosomal spiremes, 

 pairing two by two, are seen to form six threads. Now, if this 

 be the case it seems to me that we can explain such conditions 

 as Janssens has shown in figure 14 and I in figure 178, no. 9, 

 where one conjugant before conjugating with its mate seems to 

 have twisted, once in mine and several times in his. In my 

 opinion Janssens's chiasmatype theory does not explain satis- 

 factorily this phenomenon, while that of a presynapsis splitting 

 does. 



This theory appears to me based upon an unnecessary and 

 incorrect interpretation of the first spermatocyte chromosomes. 

 It fails to explain all of the phenomena of twisting in these chromo- 

 somes, and there is some evidence to show that it probably could 

 not occur. Until the theory can be established upon a more 

 firm basis cytologically, we are not justified in accepting it un- 

 reservedly in our study of problems of heredity. 



5. Linkage as shown in Y -chromosomes — a basis for coupling and 



repulsion 



Such linkage of chromosomes as Chorthippus reveals when 

 compared with Syrbula and as the species of Jamaicana, more 

 especially J. subguttata, show, furnishes a second possibilit}^ 



