CROSSING OVER AND CHROMOSOMES 103 



The next two figures (Fig. 42, a, b) show how Robert- 

 son and Wenrich interpret the crossed threads, that they 

 have observed in the spermatogenesis of some of the 

 grasshoppers. The four strands are represented as con- 

 jugating side by side in Fig. 42, a. When the strands 

 begin to open out preparatory to the first spermatocyte 

 division the two maternal separate from the paternal at the 

 ends of the tetrad, while in the middle of the tetrad the 

 opening up involves the separation of a maternal and a 

 paternal strand from a maternal and a paternal. In 

 other words, the tetrad opens up in two planes at right 

 angles to each other. This scheme also gives an appa- 



Fifi. 43. — Scheme showing crossing over involving both strands of each chromosome. 



rent crossing of the strands at the level where the open- 

 ing out in one plane passes over into the opening out 

 in the other plane, but there has been no real crossing 

 over of the strands in the sense of interchange between 

 them. Theoretically this explanation is sound, and 

 moreover seems to be supported by observations in 

 cases where the maternal and the paternal strands can 

 be identified. The results undoubtedly show that the 

 occurrence of crossed threads in cases where the split 

 occurs in two planes does not necessarily imply that 

 crossing over has taken place ; but, on the other hand, as 

 has been shown (in Fig. 41) a similar figure may also 

 necessarily result after crossing over of the threads. In 

 a word, the crossed-strand stage is not ipso facto evidence 

 that it must have come about according to Robertson's 



