No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 437 



may have been produced by the action of the preserving fluid 

 (hot aqueous solution of corrosive sublimate). 



7. Siichosiemma eilhardi (Montg.). 

 (Plate 27, Figs. 213-235.) 



The yolk changes may first be delineated, then those of the 

 nucleoli. In my paper on this fresh-water form ('95), I have 

 described the ovogenesis to some extent, and here shall follow 

 it more in detail. 



Yolk. — The yolk first appears in the cytoplasm in the form 

 of small, more or less spherical masses (Fig. 213, Yk. Bl), 

 which at first stain like the cytoplasm ; but these youngest 

 recognizable yolk balls consist of a substance in which the fine 

 granules (or nodal points of an alveolar structure) are much 

 more densely grouped than in the surrounding cytoplasm. 

 Thus the young yolk ball may be distinguished from the 

 cytoplasm proper by its greater density. A number of these 

 yolk balls appear to arise simultaneously, though in these 

 earliest, as well as in the later stages of yolk formation, a 

 successive production and metamorphosis of yolk balls take 

 place, since in all but the earliest stages of their development 

 yolk balls occur in the cytoplasm in various stages of forma- 

 tion. There is no rule as to the part in the cell at which these 

 balls are destined to arise, for they may be found anywhere 

 between the nucleus and the periphery of the cell ; the fact 

 that they first arise just as frequently at some distance from 

 the nucleus as in its immediate neighborhood shows that they 

 have no nuclear origin. An anabolic and a katabolic series of 

 changes of each yolk ball can be distinguished, and these series 

 of metamorphoses may be described in succession and termed 

 respectively the prophasis and metaphasis of the yolk balls. 



Prophasis {Yk. Bl. in Figs. 217, 218, and the median ones of 

 Fig. 215). — The progressive or anabolic changes of the yolk 

 balls consist in (i) their absorbing protoplasmic stains with 

 great intensity, so that they stand in marked contrast to the 

 cytoplasm ; and (2) in their becoming quite homogeneous in 

 structure, this homogeneity probably explainable by supposing 



