CROSSING OVER AND CHROMOSOMES 105 



division during which the chromosomes expand into a 

 sort of vesicle, as seen by comparing Figs. 44, a and 

 44, b. An optical cross section of the stages shown 

 in the last figure is represented in Fig. 44, c. An 

 older stage is seen in Fig. 44, d. The stage of greatest 

 diffusion of the chromatin material within its vesicle is 

 seen in this figure, where the outlines of each vesicle are 

 still visible. As the nucleus gets ready for another divi- 





Fig. 44. — Spermatagonial cells in the last phase of division, a, and the following resting 



stages, b, d. (After Wenrich.) 



sion the vesicles become more distinct (Fig. 45, a,h), and 

 soon a coiled thread can be seen to be present in each 

 vesicle (Fig. 45, c). As the thread thickens (Fig. 45, ^), 

 a longitudinal split appears in it, which indicates the plane 

 of division of each chromosome at the next division. 



At the last spermatogonial division, the chromosomes 

 of the two daughter nuclei form vesicles, as they have done 

 in earlier divisions (Fig. 46, a and h). But changes begin 

 to take place that carry the chromosomes through a very 

 different series of stages from those seen in preparations 



