No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
may have been produced by the action of the preserving fluid 
(hot aqueous solution of corrosive sublimate). 
7. Stichostem?na 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 (1) 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 
