416 ELMER L. SHAFFER. 



sume more and more a definite individuality, and the nucleus 

 then becomes filled with a number of very delicate leptotene 

 threads (Figs. 37, 75). These often show a distinct polarization, 

 usually being attached at one side of the nuclear membrane 

 with the free ends suspended in the nuclear sap. During this 

 stage there is also a marked tendency for the leptotene threads 

 to become associated in pairs and this pairing becomes more 

 marked in the later stages (Fig. 38). The threads are now 

 markedly polarized and it becomes quite clear that they are 

 actually pairing side-to-side (zygotene stage). This paired ap- 

 pearance of the threads is not an accidental one, for I have 

 observed it in a great many cells with great clearness. That it 

 is also not due to a longitudinal split of a single thread is evidenced 

 by the fact that the chromatin granules in homologous threads 

 do not lie at the same level. After this pairing has taken place 

 and the threads have become well polarized, they gradually 

 become shorter and thicker (Fig. 39), forming the typical pachy- 

 tene stage, the threads sometimes showing the primary split or 

 point of synaptic union. A typical pachytene bouquet stage 

 follows, in which the loops have both their free ends attached 

 at one pole of the spindle (Figs. 40, 76). Usually there is one 

 especially large loop (A A, Fig. 40) similar to the large loop 

 found in the bouquet stage in the spermatocytes, which un- 

 doubtedly represents the macrochromosome pair. Cross-sec- 

 tions of cells in the bouquet stage show the threads on end view 

 (Figs. 41, 77), and it is possible to count these. Usually there 

 are 20 such cut ends of the pachytene threads and since each 

 thread has been cut twice, we can deduce that there are ten 

 pachytene loops, and it is at once seen that this number corre- 

 sponds to the reduced number of chromosomes. 



The release from the bouquet stage sets free the thick woolly- 

 looking pachytene threads (Fig. 42) and the primary split comes 

 clearly into evidence, and often the homologous elements become 

 separated along the synaptic line. As this separation con- 

 tinues, the homologous threads become twisted about each other 

 assuming the typical strepsistene condition (Fig. 43). The 

 separation may begin at either or both ends of the threads or 

 they may retain their union at the ends and separate along their 



