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THE AMERICAN NATURALIST [Vol. LIV 



for no special comment. It ends with the significant 

 statement 



At our next meeting I shall point out that the theory of the chiasma- 

 type allows for an interpretation somewhat different from the view of 

 simple splitting of the threads in a single plane that passes through 

 the axis of the entwined threads. 



Concerning the point here raised by Janssens I should 

 like to add that the " simple interpretation" was given 

 mainly to escape the somewhat complicated scheme in- 

 volved in Janssens 's theory. In this way it was hoped to 

 avoid a too detailed account of the process that calls for 

 pictures not readily understood except by cytologists 

 familiar with the changes that take place when the twisted 

 threads shorten and move apart. Unfortunately the 

 very simplicity of the statement led one critic to infer 

 that the interpretation must be wrong because at the 

 nodal points the plane of the split appeared as though it 

 cut obliquely through each chromosome itself. To avoid 

 this I represented in later diagrams the chromosomes as 

 made up of beads, and in this way tried to show that at 

 the node each bead and its allelomorph are not divided, 

 but go each to a pole. Even this diagram may prove too 

 simple; for, if at the time of twisting each thread is also 

 split lengthwise into two strands it is possible that only 

 two of the strands fuse at each node. In Janssens 's 

 scheme to be described below this secondary doubling of 

 each thread is seen to be an important factor in the 

 situation. 



Janssens states that the matter is not simple : 



The loops (Fig. IB) and the half loops (Fig. 1 C) that produce the 

 chiasma lead to profound modifications in the twisted threads. These 

 modified inns are already indicated in the prophases, but they only 

 become evident in proportion as the dyads ripen and prepare to place 



state nevertheless that the chiasma segments are placed in planes per- 

 pendicular to the segments adjacent to them as indicated in diagram 

 IT, Fig. 1. Once this fact is clearly seen it is not essential to add much 

 to Morgan's phrase, since it expresses what really takes place. It need 

 only be said: (1) In both maturation divisions a cleavage takes place 



