BIGELOW: NUCLEAR CYCLE OF GONIONEMUS MURBACIIII. 303 



differs sharply from that seen in the spermatocytes, and forms, therefore, 

 an excellent criterion for determining whether any particular cell in the 

 resting stage is a spermatogonium or a spermatocyte. Size, although a 

 valuable index, is in this question much less reliable ; and position, 

 whether close to the mesogloea or more peripheral, is not a safe indica- 

 tion, since groups of cells in different parts of the gonad develop at 

 different rates. 



The cytoplasm of the spermatogonia stains strongly with plasma dyes, 

 is densely and evenly reticulate, contains no vacuoles, and shows no 

 archoplasmic structures that are clearly demonstrable. In fact, in no 

 medusan cell in the resting stage (except, of course, the spermatid) have 

 I been able to distinguish idiozome, Nebenkern, or other permanent cell 

 organ of similar nature. The question of the presence or absence of 

 the centrosome is, however, less easily answered. In a considerable 

 number of cells (Fig. 19) I have been able to distinguish near the 

 nucleus a minute black spherule, frequently surrounded by a clear area, 

 in which the reticulate appearance of the cytoplasm is not visible. At 

 first sight this body might well seem to be the centrosome, but it is by 

 no means certain that such is the case. In many cells (Fig. 16) there is 

 visible a black granule which, however is surrounded by undifferentiated 

 cytoplasm. In the great majority of cells no such structure is to be 

 seen, while, on the other hand, the cytoplasm sometimes contains several 

 dark granules of different sizes (Fig. 16). Furthermore, the presence 

 or absence of the homogeneous area above mentioned seems to be of 

 little significance, for I have often observed similar appearances at the 

 periphery of the large metaplasmic masses to he described presently. 

 Finally, I have not been able to trace with certainty a genetic connection 

 between any of these structures and the centrosomes first demonstrable 

 beyond doubt in the early metaphase of division. It might be reason- 

 ably supposed that the sure detection among the various cytoplasmic 

 granules of so minute a body as the centrosome would be very difficult. 

 It should, however, be borne in mind that in the spermatid, as later 

 described, there is no stage when the centrosome is not clearly visible, 

 even though it is very small and imbedded in cytoplasm of the same 

 granular appearance as that of the spermatogonia. We must conclude, 

 then, that the presence of a centrosome in the resting stages of the 

 spermatogonia is at best doubtful, and if we assume its existence it must 

 be on ct priori grounds rather than from any actual internal evidence 

 affoi'ded by the cells. I lay stress upon this for the sake of emphasizing 

 the difference exhibited by somatic cells, spermatogonia, and spermato- 

 cytes on the one hand, and spermatids and spermatozoa on the other. 



