74 bulletin: museum of comparative zoology. 



erably greater than in the stage last described. The anastomosis is 

 to be seen more particularly at the sides of figure 25. Through the 

 middle of this figure the individual spiral threads seem to be more 

 easily distinguishable, and I am inclined to believe that the two which 

 stain more deeply than the others are the members of the A pair of 

 chromosomes. The stages including and following this reticular 

 stage are hard to represent in a drawing of the kind employed, owing 

 to the difficulty of portraying in their natural relations the parts seen 

 at different planes of focus. Careful study has always convinced me, 

 however, that the uncoiling and elongating threads are single, con- 

 tinuous, and not united into an indiscriminate network. I have 

 selected in figm-es 26 and 27 views favorable for di'awing where some 

 of the threads, at least, are definitely separate and continuous across 

 the diameter of the nucleus. 



At the stage represented in figure 28 (Plate 3) the unwinding of the 

 coiled threads has been completed, but the threads have as yet no 

 definite orientation. At the somewhat later leptotene stage shown in 

 figure 29 the threads are finer and less homogenous than in the earlier 

 stage, the substance of the thread seeming to have become more 

 distinctly differentiated into a linin fiber and cliromatic granules, the 

 latter scattered at irregular intervals along the fiber. Moreover, in 

 this later stage the threads appear to be definitely oriented, with one 

 end attached at the proximal pole of the nucleus. The threads then 

 take a course thi'ough the center of the nucleus or near its periphery, 

 extending wholly or partly across and then turning back with a wide 

 curve. 



2. The zygotene stages. — In figure 30 (Plate 3) some of the threads 

 are double, others are single, and it would be difficult to decide from 

 a casual examination of this stage alone whether or not the double 

 threads had arisen by a splitting of the single ones. In the case of 

 one or two of the double threads, however, as may be seen at the left 

 side of the nucleus, the double condition does not continue throughout 

 the whole length, but towards the distal end of the nucleus the thread 

 is seen to branch into two single threads. I interpret tliis branching 

 thread as one in which the parallel conjugation has not yet been 

 completed. Another instance of the same kind may be seen in figure 

 31, which represents a stage somewhat more advanced than that of 

 figure 30. These appearances lead me to believe that conjugation 

 begins at the side of the nucleus corresponding to the proximal ends 

 of the leptotene threads, and proceeds gradually toward their distal 

 ends. It is further evident from these figures that conjugation is 



