LINKAGE 



393 



Similarly unsatisfactory is the cytological evidence for the breaking 

 and reunion of the chromatin threads required by the crossing over hypo- 

 thesis. Since the hypothesis was put forward by Janssens (1909) adequate 

 and convincing descriptions of this process have been singularly wanting, 



A' 



FIG. 156. Diagrams illustrating various possibilities concerning the compound ring 

 tetrads in Orthopteran spermatocytes, following the outlines of Janssens's figures, but 

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

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

 cross-overs in B 1 and C 1 . A, the compound ring as conceived by McClung, Robertson, etc., 

 with the four resulting chromatids at .4. 1 (no cross-overs). B, a compound ring, such as 

 might follow a two-strand cross-over at each node, giving the results shown in B 1 . C, a 

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

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

 The result (C 1 ) is four classes of chromatids, as shown in C 1 . (Figure and legend from 

 Wilson and Morgan, 1920.) 



particularly in those cases in which experimental results would make its 

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

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

 some tetrads in the spermatogenesis of certain insects may be interpreted 



