OOGENESIS IN THE WHITE MOUSE 273 



pearance (to judge from their figures) is an artifact, due to over- 

 fixation with the osmic acid fixer used (Flemming's). The 

 cells are described as being in the germinal epithelium, and I 

 have found that over-fixation is regularly the fate of the cells 

 in the outer laj^ers of the ovar}-. 



In the mouse, the cells pass from the 'protobroque' to the 

 'leptotene' stage', which is of short duration. The chromatin 

 net- work gradually becomes heavier in places and breaks down 

 in others, so that the result is the formation of long slender 

 bands or threads, connected by delicate cross-bars (figs. 15, 16). 

 These cross-bars disappear later and the chromatin is then ar- 

 ranged as a tangle of long slender threads (fig. 17). At least 

 one nucleolus is present, frequently hidden in the knot of chroma- 

 tin bands. An idiosome is not yet distinguishable. It is diffi- 

 cult to say whether the chromatin threads have any definite 

 arrangement; in some cells they seem to have more or less the 

 form of a horse-shoe, with their ends all toward the same side 

 of the nucleus. This disposition, however, does not seem to be 

 universal. Cells in this phase are usually found in the germinal 

 epithelium, but many are also encountered beneath this in the 

 ovary. 



This stage soon passes into the following in which the chromatin 

 threads undergo a contraction to one side of the nucleus — - 

 synizesis (fig. 18). This contraction is not extreme in the mouse 

 and the chromatin bands can usually be distinguished, a few 

 extending out of the tangle. The nucleoli are hidden among the 

 threads and it is difficult to make them out; one at least, however, 

 is present at this time. No idiosome was distinguished, so it is 

 hard to say with certainty w^hether the contraction of the chroma- 

 tin is toward that side of the cell. The mitochondria have in- 

 creased in number and are massed around the nucleus. Cells 

 in this stage are most frequently found in the ovary under the 

 germinal epithelium. 



The next stage, 'pachj'tene,' is to a certain extent synchronous 

 with the preceding. During synizesis the long slender threads 

 of chromatin begin to shorten and thicken. As this progresses, 

 they assume, at the same time, a moniliform appearance, as if 



