448 BENJAMIN H. PRATT AND J. A. LONG 



of days before birth. The orientation of the threads first dis- 

 cernible in the leptotene stage is distinctly more advanced. 

 Figure 11 is a lateral view while figure 10 is a polar view of a 

 nucleus in the late synaptene stage. In figure 10 the nucleus is 

 viewed from the pole opposite the so-called centrosome. It is 

 clearly evident from figure 11 that the loops of the synaptene 

 threads are of different lengths, some of them extending barely 

 half as far out into the nucleus as do the others. Counts of the 

 loops as seen in polar views or sections have not been made at 

 this stage. Moreover, since it has not been possible to deter- 

 mine the number of leptotene threads, a comparison of the num- 

 bers in these two stages cannot be made as evidence of pairing. 

 Nevertheless a consideration of the sizes and structures of the 

 leptotene and synaptene threads give reasons for beheving that 

 the latter are derived from the former. A comparison of figures 

 8 and 9 with 10 and 11 makes it evident that the synaptene 

 threads are about double the width of the leptotene. The synap- 

 tene is further clearly composed of distinct moniliform halves sep- 

 arated by a longitudinal split (fig. 10). Each half of the double 

 synaptene thread is very similar to the leptotene threads, and 

 bears much the same relation to the other half which some lepto- 

 tene threads have to other leptotene threads. That is, the spin- 

 dle shaped and laterally flattened enlargements in each half are 

 usually opposite and nearly or quite in contact with those of the 

 other half. Also the portions of each moniliform half, between 

 the deeply staining enlargements, are almost or quite devoid of 

 chromatin. It seems then that each of these synaptene threads 

 was formed by the side by side approximation of two leptotene 

 threads. 



Throughout this period as in the preceding ones a chromatin 

 body in any way simulating an accessory chromosome is not 

 evident. 



THE PACHYTENE STAGE 



The disappearance of the longitudinal division in the synaptene 

 thread marks its transformation into the typical thread of the 

 pachytene stage. This stage begins two days or thereabouts 

 before birth and persists for about a day thereafter. The ori- 



