No. 632] CHI ASM A TYPE AND CROSSING OVER 215 



in ring Bb the dark double thread (half ring) at the right 

 separates from the light double half at the left. This is 

 a reduction division for this segment. On the other hand 

 in the ring Cc the division plane separates equationally 

 the halves of the dark and of the light half rings. Cross- 

 ing over takes place at the node between ring Bb and Cc, 

 and at the node between the rings Cc and Dd there is an- 

 other crossing over between the other two strands. Gen- 

 eralizing the result it may be said that crossing over of 

 two of the strands takes place at each node. In the sec- 

 ond division, that is supposed to take place here in the 

 plane of the paper, there is assumed to be no further 

 crossing over in either of the halves that have resulted 

 from the first division. 



Janssens points out that on this new scheme there is 

 only half as much crossing over as on the scheme repre- 

 sented in our older diagram (1913) ; but it is obvious that 

 this is only because in the latter whole chromosomes 

 (each potentially or actually made up of two strands) 

 are represented as crossing over at each node. If, how- 

 ever, we compare this latest scheme of Janssens with the 

 figures that we have now recently published ("Physical 

 Basis of Heredity," 1919) in which, following some of 

 Janssens 's earlier diagrams only two of the strands 

 cross over at each node, it is perfectly clear that these 

 later schemes of ours give the same number of cross- 

 overs per complete twist as do Janssens 's present dia- 

 grams. 



It may, therefore, not be without interest to compare 

 Janssens 's latest scheme with the one I have recently 

 suggested in my book on the "Physical Basis of Hered- 

 ity" (p. 105) where a figure is given that suggests an ex- 

 planation of the opening out of a twisted conjugated 

 thread in rings that lie in different planes. This figure 

 is here reproduced, Fig. 8 A-D, modified only so far as 

 to make it comparable with Janssens 's new diagram. In 

 A the two split threads are represented as looping or 

 overlapping in an open spiral (an earlier stage than 



