42 THE THEORY OF THE GENE 



There are many published figures of the chromosomes 

 twisted about each other. But in some respects this evi- 

 dence proves too much. For instance, the most familiar 

 and best ascertained stage, where twisting is obviously 

 present, is found at the time when the conjugant pairs are 

 shortening preparatory to entrance into the equator of 

 the spindle (Fig. 27). The usual interpretation of the 



a 



Fig. 27. 

 The late twisting of the thick threads (chromosomes) just before 

 they enter the spindle of the first maturation division of Batracho- 

 seps. (After Janssens.) 



twisting at this stage is that it is in some way connected 

 with the shortening of the two conjugants. There is noth- 

 ing in these figures to show that this leads up to inter- 

 change. While it is possible that some of the cases of this 

 kind may be due to an earlier twisting of the threads 

 about each other, yet the persistence of the spiral would 

 rather indicate that crossing-over had not taken place, 

 for this would undo the twist. 



If we turn next to the published drawings of the earlier 

 stages we find a number of cases in which the thin threads 

 (leptotene stage) are represented as though twisted 



