196 



THE AMERICAN NATURALIST 



[Vol. LIV 



papers that lie does not consider the diagrams to have 

 misrepresented the gist of the matter. 



So far as can now be judged, Janssens's latest studies 

 add nothing to his paper of 1909 that is new in principle. 

 They are in the main an elaboration of his earlier con- 

 clusions concerning the double-ring and double-cross 

 types of tetrads, which were illustrated in his former 

 work by diagrams XI, XVI, XIX and XX. These two 

 forms of tetrads are closely related, and each of them 

 shows in the prophases of meiosis a two-strand chiasma 

 —i. e., two threads which seem to pass over from one 

 synaptic mate to the other, crossing each other midway 

 between them, as in Fig. 5, I or 5, III— such as formed 

 the main basis of the original chiasmatype theory. It 

 may be needless to describe these tetrads, which are per- 

 fectly familiar to cytologists, but for the sake of clear- 

 ness I will briefly review their composition as now gen- 

 erally understood. 



