110 



PHYSICAL BASIS OF HEREDITY 



threads furnish no proof that an interchange must have 

 taken place earlier, but neither does it furnish any evi- 

 dence that interchange had not taken place. For example, 

 the most obvious interpretation of Fig. 48, 2 d is that 

 the upper end of the tetrad has separated in the plane 

 of the secondary split (in anticipation, as it were, of the 

 separation about to take place in this plane), and has sepa- 

 rated in the lower part of the same tetrad in the plane of 

 the primary split. This interpretation does not involve 

 any real crossing over in the sense that the two crossed 

 threads had previously broken and interchanged, as Jans- 









Fia. 49. — The same chromosome pair in conjugation from thirteen different cells. (After 



Wenrich.) 



sens' chiasmatype assumes on the ground that the two 

 granules (threads) in contact at the upper end of the 

 tetrad must be related to each other in the same way as 

 are those further back in the tetrad. 



This last assumption is the foundation of Janssens' 

 view, but has no longer sufScient evidence to support it, 

 even though none opposes it. Nevertheless, it should be 

 clearly understood that evidence such as this, derived 

 from Wenrich 's results, can not possibly be held to show 

 that an earlier interchange or crossing over has not 

 occurred. If it had, such a figure as this (c) would, as 

 explained above, be a consequence to be expected. 



The constancy of the beading of the chromosomes in 

 each individual is most remarkable. Its significance for 



