254 WM. REES B. ROBERTSON 



4. On the chiasmatype of Jansse7is 



In the first spermatocyte chromosomes of Syrbula and Chor- 

 thippus (figs. 150, nos. 6, 8, 11; 152a; 156b; 174, 7-11; 176; 

 178) I have bodies smiilar to those of Triton and Batrachoseps 

 upon whose structure Janssens ('09) bases his 'theory of the 

 chiasmatype.' Morgan ('13, '14) has made much use of this 

 theory in explaining the failure of the linkage of characters 

 usually coupled together in transmission to offspring. Janssens 

 used it to furnish a basis in the germ cells to account for the 

 occurrence of a larger number of allehnorphs than there are 

 pairs of chromosomes by which they may be borne. 



According to this theory the chromosomes which pair side 

 by side in synapsis twist about each other in an irregular spiral 

 manner on coming out of this stage. Before the members of 

 a pair separate each may be seen to be split longitudinally, so 

 that the tetrad is made up of four longitudinal strands. Jans- 

 sens has noted strands crossing over from one conjugant chromo- 

 some to the other (his text, fig. XXII) and upon the basis of 

 this observation concludes that when such paired chromosomes 

 later complete their disjunction they represent combinations 

 different from those present before synapsis due to the estab- 

 lishment secondarily of these 'cross-over' connections. 



If Janssens assumes that the breaking and secondary fusion 

 of those crossed-filaments nearest each other (text figs. XIII- 

 XV) takes place after the chromosome has reached the stage 

 represented by his figures 1 to 13 (pi. I) etc. (stage ^K or '^' 

 of Wilson), I believe that he has been mistaken in his inter- 

 pretation of the form of the chromosome (text fig. XI) upon 

 which he has based his theory, but if he believes this secondary 

 fusion takes place much earlier, following the amphitene and 

 preceding the pachytene stage, then I have little to say. As 

 will be seen from his figures (text figs. I, IX, XI, XXI, XXVII), 

 he believes that the chromosomes conjugating side by side 

 (each split longitudinally) should normally cross each other en- 

 tire, i.e., with both filaments, at the nodes (N, text fig. I). He 

 finds, however, that instead of both filaments of a segment 



