No. 632] CHIASMATYPE AND CROSSING OVER 201 



precision especially by Eobertson ( '16) and by Wenrich 

 ( '16, '17). None of these observers, it is true, has traced 

 the history of the rings in every detail ; hut their results, 

 so far as they go, are entirely in harmony with the better 

 known history of the single rings and double crosses, 



both of which offer essentially the same problem as the 

 compound rings. These various forms of tetrads arise 

 from a diplotene thread that is at first longitudinally 

 double and sooner or later longitudinally quadripartite 



