No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
433 
in the size of the nucleoli, a decrease in their number, and the 
gradual migration of them towards the periphery of the nucleus. 
At the beginning of this period (Figs. 290, 291, 293, 294), the 
nucleoli are quite evenly distributed throughout the nucleus; at 
its close they are mainly peripheral in position, near the nuclear 
membrane (Fig. 297). The increase in the size of the nucleoli 
is due, in some part at least, to the coalescence of every two or 
three neighboring ones, and such juxtapposed groups of two 
or three nucleoli may be often found (Fig. 294). None of the 
nucleoli contain vacuoles. 
Fourth nucleolar stage. — Now we find unstaining globules 
or vacuoles reappearing in the nucleoli, and there may be either 
a single large one to each nucleolus, or a number of smaller 
ones ; the large one is probably formed by the coalescence of 
smaller ones. Almost all the nucleoli are in contact with the 
nuclear membrane, often flattened against it (Fig. 299). They 
have become larger than in any preceding stage, and less 
numerous, but are now quite unequal in size. This stage may 
mark the commencing degeneration of the nucleoli, though I 
have observed no evidences of a commencing fragmentation. 
At the beginning of the first stage the nuclear sap never 
stains ; but at the end of this period, when the nucleoli have 
become more numerous, it stains very noticeably with eosin 
(Fig. 286), which would point to a solution of nucleolar sub¬ 
stance in the nuclear sap. 
6 . Zygonemertes virescens (Verr.) Montg. 
(Plate 27, Figs. 236-248.) 
Yolk. — In only two cases out of the numerous egg cells 
examined (three individuals of this worm were sectioned) have 
I seen yolk balls, so that the formation of yolk balls must be 
regarded as abnormal, if not pathological ; in this species the 
yolk arises as minute yolk spherules in the cytoplasm (Fig. 
246), without (except in the cases noted) a yolk-ball stage 
being passed through. These minute globules stain at first 
very faintly, and when they first appear are isolated from one 
another. There is no given point in the cytoplasm where they 
