104 PHYSICAL BASIS OF HEREDITY 



scheme. It should also be observed that the scheme rests 

 on the assumption that no twisting has preceded the stage 

 of the crossed threads, or, if such has taken place it 

 has no relation to the resulting chiasma. Yet crossing 

 of the threads is an observed fact. 



A third scheme (Fig. 43, a, h) makes both maternal 

 strands interchange with both the paternal ones. This 

 scheme has at least one formal advantage over the other 

 two in that it represents the four strands, after crossing 

 over, as in position to lie side by side in the tetrad, so that 

 the two longitudinal splits that reappear later lie in the 

 same plane throughout their length. This seems more in 

 accord with many of the observations that are recorded. 

 If, during the following stages, the tetrads open out by the 

 separation of the maternal from the paternal strands the 

 crossed threads that result correspond to those in the first 

 scheme (Fig. 41). At present it is not possible to decide 

 between these different modes of representing crossing 

 over. They may all occur. Their discussion shows little 

 more than certain possibilities involved in the situation. 



Details of Spermatogenesis 

 Some of the stages in the spermatogenesis of a grass- 

 hopper, Phrynotettix, as described by Wenrich, are shown 

 in the following figures. The material furnishes certain 

 details concerning the ''resting stages" of the nuclei 

 preceding synapsis more completely than any other, and 

 it serves also to illustrate clearly the relationship of the 

 chromosomes to the vesicles into which they pass (or 

 which they form) during the resting stages. The figures 

 also show how the threads emerge from the vesicles in 

 which they appear to have been contained during the 

 resting stages, and how the opening out of the tetrads 

 in two planes gives the appearance of chiasma accord- 

 ing to Wenrich. 



During the time when the germ-cells are increasing in 

 number by division there is a resting stage after each 



