344 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



any mitosis, but leads once more through regressive changes to a "rest- 

 ing condition " — is one of great theoretic interest, I will summarize 

 the lines of argument which make it, to my mind, the only reasonable 

 one. Negative evidence, it is true, is never altogether satisfactory ; the 

 fact that no mitotic stages are found being of course not conclusive that 

 none occur; there is, however, certain circumstantial evidence of this 

 which nearly amounts to proof. 



In the first place, in every mitosis in Gonionemus, although early 

 prophases may be rather rare, as in the somatic cells and in oogonia, 

 mitotic figures are from their conspicuous nature always easily found, 

 furthermore, the late prophases are quite as abundant as the earlier ones. 

 For the sake of comparing the abundance of the "prophases" of the 

 oocyte with that of other generations, I have counted them, and have so 

 far noted three hundred cells in stages between those represented in 

 Figures 122 and 127. They are thus very much more numerous than 

 corresponding phases in either oogonia or somatic cells. Yet I have not 

 found a single case where the plasmosome is not clearly visible, where 

 the nuclear membrane had broken down, or where, in short, the course 

 of events had progressed any further than the typical " segment stage " 

 represented in Figures 125, 126, 127. The evidence of the persistence 

 of the nucleolus I consider very important, since in all mitoses stages 

 immediately after its disappearance are quite as numerous as those just 

 prior to that event. Still further, although we might perhaps assume 

 thafthe lack of mitotic figures was due to the great rapidity of this mi- 

 tosis, we can hardly suppose this to be true of the whole series of im- 

 portant changes leading from the spireme stage to the appearance of 

 chromosomes in the metaphase. It has been suggested that we might 

 consider the mitosis, which I have already described as belonging to the 

 last oogonial division, as being in reality the outcome of the conditions 

 now under consideration. But this is, I think, conclusively negatived 

 by the following facts. In the first place, the sequence of the different 

 steps, from the early prophase of the oogonium (Figs. 110-114) to the 

 appearance of the chromosomes in the equatorial plate, is so close and so 

 easily traced in detail that I have no doubt that they do actually suc- 

 ceed one another. For the evidence of this I refer the reader to the de- 

 scription and figures of the oogonial mitosis (pp. 336—341). Furthermore, 

 the oogonia in mitosis very closely resemble somatic cells, whereas the 

 oocytes in the apparent prophase differ very markedly from them both 

 in size and nuclear structure. In addition, the position of the different 

 types of cells in the gonad is of some importance in this connection. As 



