LINKAGE 





Similarly unsatisfactory is the cytological evidence for the breaking 

 and reunion of the chromal in 1 hreads required by 1 !)<■ crossing over hypo- 

 thesis. Since the hypothesis was pu1 forward byJanssens L909) adequate 

 and convincing descriptions of this process have been singularly wantii 



A 



B 



KJ 



A 1 



I 

 I 



I 



«• ft 



v B 



e C 



t I 



B 1 



r\ 



I 



I 



I 



B 



I 



Fig. 156. — Diagrams illustrating various possibilities concerning tin- compound ring 

 tetrads in Orthopteran spermatocytes, following the outlines <>f Janssens'a figures, bu1 

 showing also the relations of the chromatids. At the left in each of the upper figures i- the 

 longitudinal tetrad-rod from which the riim-series arises, showing results of assumed early 

 cross-overs in B l and C 1 . A, the compound ring as conceived by Mc< Hung. Robertson, etc., 

 with the four resulting chromatids at .1' (no cross-overs) . H, a compound ring, Buch as 

 might follow a two-strand cross-over at each code, giving the results shown in IV. I 

 compound ring giving the results shown in Janssens'a diagrams, resulting from a two- 

 strand cross-over between two pairs of threads, in regular alternation at successive nodes. 

 The result (i' x ) is four classes of chromatids, as shown in C l . (Ft<rttr< ami legend from 

 Wilson and Morgan, 1020.) 



particularly in those 1 cases in which experimental results would make its 

 establishment most desirable. Wilson (19126), Robertson (1916), and 

 Wenrich (1916, 1917) point out thai the figures formed by the chromo- 

 some tetrads in the spermatogenesis of certain insects may be interpreted 



