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THE 



AL1ST [Vol. LIV 



stage the essential step in the breaking and reunion of the 

 strands remains to be seen when his new results are pub- 

 lished. 



Until Janssens publishes a full statement as to how he 

 supposes the crossing over at the nodes to take place, 

 whether at the time when the looping of the threads is 

 present, or at an earlier stage, it is hazardous to make too 

 detailed comparisons, but one relation should not pass 

 unnoticed. In Janssens 's figure four rings seem to be 

 involved in one complete twist of the two chromosomes. 

 In order to place these rings in> such a position that a 

 single (vertical) plane can sunder successive rings trans- 

 versely and longitudinally in alternation, the rings must 

 be turned so that two are exactly vertical and two are 

 horizontal. A spiral relation of the threads can not be 

 brought into this relation unless the threads first untwist. 

 How this can be done is shown by a comparison of Fig. 7 

 with Fig. 8. In Fig 7 A, as explained, two chromosomes, 

 each of two strands, are represented as looped around 

 each other in an open spiral. In the middle of the spiral 

 the two inner strands that touch are represented as fus- 

 ing and reuniting to give the cross-over, and near the 

 ends, where the threads cross again, the other two strands 

 fuse, break, and reunite to cross over, Fig. 7 B. The 

 threads are then represented as flattening against each 

 other, still keeping their spiral configuration. When they 

 open out again, by a reductional separation of the seg- 

 ments, Fig. 7 D, the rings are formed, and if the threads 

 are still represented as keeping their spiral configura- 

 tion no single plane, as explained, will separate them 

 without cutting some of the strands. But if when stage 

 C is reached in Fig. 7 the threads straighten out as they 

 condense (i. e., if they untwist) the result will be that 

 shown in Fig. 8 A. If now the threads open out by a re- 

 duction division in each segment, the resulting figure will 

 be like that shown in Fig. 8 B. This figure is the same as 

 that of Janssens, and the halves can be separated in one 

 plane, as he explains. We may conclude then, if the con- 



