112 PHYSICAL BASIS OF HEREDITY 



out of it with at least the same size they had on entering 

 it. Each pairing chromosome maintains its distinct indi- 

 viduality during this period. This is opposed to the 

 idea of Janssens ( ^09) and Morgan ( '11), as expressed in 

 the theory of chiasmatype. In their theory they assume 

 that homologous chromosomes in parasynapsis twist about 

 each other and fuse. On splitting, a plane passes down 

 the fused body, regardless of the previous spiral fusion 

 plane, resulting in two daughter chromosomes which may 

 not be identical with the tw^o chromosomes which entered 

 the process. Each new one may contain parts of both 

 original chromosomes. If such had been the case, the 

 separation or formation of a short and a long chromo- 

 some out of the first chromosome w^ith such regular- 

 ity of size, etc., as w^e have sho^vn, could not have 

 occurred.'' On the contrary, even if crossing over had 

 occurred within the region where the short and the 

 long pieces came together, the separation would be 

 expected still to be exactly that described by Robertson; 

 for the genetic evidence points very clearly to the con- 

 clusion that the interchange involves exactly equal and 

 opposite parts. There is no reason to suppose that 

 regions outside the conjugating region would be affected ; 

 on the contrary, all the genetic evidence would lead us to 

 expect no such effects. 



SUMMAKY OF EVIDENCE 



If we have found Janssens' evidence inadequate as a 

 demonstration of crossing over, what other evidence is 

 there in the history of the chromosome to which an appeal 

 can be made! First, there is the undisputed fact that at 

 the time when the chromosomes come together they spin 

 out into long, thin threads which, as they meet, lie over 

 and under each other, so that the line of fusion is in a 

 spiral plan. Later, when the fusion is complete, it is no 

 longer possible to follow the plane of union, but unless 

 the chromosomes slip around each other after crossing 



