114 PHYSICAL BASIS OF HEEEDITY 



In the female of some insects, amphibians, selach- 

 ians and annelids, the thin-thread stages in the form of 

 U-shaped loops have been described — stages that are so 

 much like those of the male that the argument for one 

 would seem to extend to the other. But again this proves 

 too much, and we have yet to learn what cytological dif- 

 ferences exist in cases where crossing over occurs in one 

 sex and not in the other. On the w^hole, then, w^hile the 

 genetic evidence is favorable in all essentials to the theory 

 of interchange between homologous chromosomes, it must 

 be confessed that the cytological evidence is so far behind 

 the genetic evidence that it is not yet possible to make a 

 direct appeal to the specific mechanism of crossing over on 

 the basis of our cytological knowledge of the maturation 

 stages. The idea that the chromosomes disappear as such 

 and go into some sort of suspension during the resting 

 stage is an old idea. 0. Hertwig thought that the chromo- 

 somes did actually ' ^ dissolve ' ' at this time and ' ^ recrystal- 

 lize" at each division stage. Goldschmidt elaborated a 

 view of crossing over that rests on the assumption that 

 the homologous genes are set free in the resting nucleus 

 and may become interchanged during reconstruction. 

 Aside from certain inherent contradictions in Gold- 

 schmidt 's scheme (the most obvious ones have been 

 pointed out by Sturtevant and by Bridges), it stands 

 in contradiction to the one most certain fact that we 

 know about crossing over, viz., that not single genes 

 but whole blocks of genes are involved — in fact, the most 

 common sort of interchange involves the two entire pieces 

 of each chromosome. 



The general idea that the genes become dissociated 

 during the resting phases is disproven by the way in 

 which they come together. The genetic evidence from 

 Drosophila shows that when crossing over occurs, let us 

 say at the middle of the chromosome, aU of the genes of 

 each half of each pair hold together — and exchange as 

 large pieces. Now if the genes are dissolved at each rest- 



