BIGELOW : NUCLEAR CYCLE OF GONIONEMTJS MURBACHII. 337 



is, however, of theoretic importance, as will appear later, to determine 

 definitely the relationships of the few division stages which do occur. 



The history of the chromatin exhibits no important differences from 

 that of the ordinary somatic cells, although from the small size and 

 general arrangement of the chromosomes, it is in the later stages easily 

 distinguished from that of the latter. 



The resting oogonia (Plate 5, Fig. 109) are similar in appearance to 

 the spermatogonia. The nucleus, which measures about G /x in diameter, 

 contains as a rule a single large nucleolus, from which there radiate 

 throughout the nucleus the threads of a delicate linin reticulum with 

 prominent karyosomes at its nodes. The nucleolus itself shows a deeply 

 staining external shell enclosing a paler central mass, an appearance with 

 which we are familiar in so many other cell generations in Gonionemus. 

 It is no doubt of composite nature. The remainder of the nuclear 

 space, as in other resting nuclei, is filled with rather dense and dis- 

 tinctly granular karyoplasm. During the "resting period" which pre- 

 cedes the last mitosis, the oogonia undergo considerable growth, and 

 previous to the commencement of the prophase the cytoplasmic body of 

 the cell (Fig. 109) is of comparatively large size and frequently contains 

 one or more of the deeply staining metaplasmic masses so common in 

 the spermatogonia. But, as in the latter case, these may often be absent. 

 No archoplasmic structures are present, nor can the presence of a cen- 

 trosome be certainly demonstrated. At this time the staining reactions 

 of the oogonia are those characteristic of resting cells in general, the 

 cytoplasm, karyoplasm, and karyosomes showing a basic, the nucleolar 

 shell alone an undoubted acid reaction. 



The general course of the prophase changes simulates very closely that 

 of the entoderm cells. The nuclear net thickens and stains more deeply, 

 while the karyosomes at its nodes grow more prominent (Fig. 110). 

 The later changes are less easy to study than is the case in spermato- 

 genesis, for the reason that it is necessary to trace them chiefly or en- 

 tirely in sections, — a method, much less satisfactory when dealing with 

 such structures as chromatin segments, than the isolation method, which 

 allows of the examination of entire nuclei. The reason for this necessity 

 is twofold. In the first place, dividing oogonia are so rare that it is 

 difficult to find them at all in isolation preparations, and in the second 

 place, their nuclei so closely resemble those of the entoderm cells in size 

 and general appearance that there is great danger of confusing the one 

 with the other. 



With the increased density of the nuclear net the nucleus loses its 

 VOL. xlviii. — no. 4 22 



