No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 427 



As we had concluded for the preceding species, so also in the 

 present and in the species of nemerteans yet to be described, 

 the nucleoli are in all probability accumulations within the 

 nucleus of a substance taken up from the cytoplasm, this sub- 

 stance being related to that which constitutes the yolk balls. 

 In the least mature germinal vesicles of T. catenulatiim we 

 found one or two very large, lightly staining nucleoli ; these stain 

 in the same way and show the same structure and degree of 

 refraction as do the daughter yolk balls (Figs. I07 and 116). 

 Further, I have noticed in the cytoplasm small yellowish spher- 

 ules (yolk-ball fragments) which are in every way similar to the 

 smaller nucleoli, and quite frequently I have observed one or two 

 of them pressed so close against the outer surface of the nuclear 

 membrane as to cause a depression of the latter (Figs. 112 and 

 118). In other words, it would seem that the substance of 

 some of the yolk-ball fragments is taken into the nucleus and 

 in the latter is re-formed into nucleoli. As long as yolk balls 

 or their fragments are found within the cytoplasm lightly 

 stained nucleoli of approximately the same dimensions as these 

 may be seen in the nucleus. I have never seen a pore in the 

 nuclear membrane through which a yolk-ball fragment could 

 penetrate, though this membrane sometimes appears to be 

 thinner at the point of contact with a yolk-ball fragment than 

 at other points in its circumference. But in the third stage, 

 when all yolk balls and their fragments have disappeared and 

 the whole cytoplasm is thickly filled with their derivatives, the 

 mature yolk spherules, large, faintly staining nucleoli, are no 

 longer present in the nucleus, but the smallest nucleoli present 

 at this time resemble in form, size, and stain, the yolk globules. 

 Therefore we must conclude that the young, small nucleoli 

 which first appear about the end of the third nucleolar stao-e 

 represent mature yolk spherules, or at least that the substance 

 of the two is equivalent. While the nucleoli of the first gener- 

 ation (formed in the first stage) are commencing to degenerate, 

 new nucleoli of a second generation begin to arise in the 

 nucleus, and the latter, which may serve as nourishment for 

 the chromatin threads, differ from the former genetically, in that 

 they are not assimilated portions of yolk-ball fragments, but 



