No. 2 .] COMPARATIVE CYTOLOGICAL STUDIES . 421 
nucleoli. The cytoplasm, when the yolk first arises in it, 
stains with haematoxylin (with the double stain of this and 
eosin); this blue stain of the cytoplasm I have noticed to be 
characteristic for the cytoplasm of many immature ova, while 
the cytoplasm of somatic cells usually stains with eosin. The 
yolk first appears in the form of large yolk balls (Figs. 144 and 
145, Yk. Bl.)y as they may be termed; the number of these 
balls varies in cells of the same size, as well as in those of 
different dimensions, and they appear to be produced succes¬ 
sively in a cell, until at the end of the third nucleolar stage they 
all have disappeared, having given place to the mature yolk 
spherules. They arise in the cytoplasm at no fixed point, 
though usually at some distance from the nucleus ; it is hardly 
necessary to state that they stand in no genetic relation to the 
nucleus, either in this or in the other nemerteans studied. 
The yolk balls are at first dense and homogeneous, and stain 
intensely with eosin ; the size that they may attain while still 
homogeneous is very variable. Subsequently they become 
vacuolated, even sometimes granular, and different portions of 
the same ball may stain differently, which shows that both a 
chemical and a physical change takes place in their substance. 
Finally, they fragment into unequal sized granules, which stain 
less deeply, and then these latter split up further, until the 
ultimate yolk spherules ( Yk. Gl.) are produced. In the largest 
ovarial eggs all the yolk balls have disappeared (they linger 
longest at the periphery of the cell), the cytoplasm being 
densely filled with the yolk spherules. In some cases yolk 
balls lie in the cavity of the gonad (Fig. 155), and these are 
probably derived from degenerated ova. 
The following facts show, I think, that the nucleoli stand in 
a genetic connection with the yolk substance. The nucleoli 
stain in the same way and have in other respects the same 
appearance as the smaller fragments of the yolk balls and as 
the mature yolk spherules (Figs. 144-146). Fragments of 
• yolk balls occur frequently in close contact with the outer sur¬ 
face of the nuclear membrane. Now since the nucleoli first 
appear in contact with the inner surface of this membrane, the 
conclusion is plausible that the nucleoli represent portions of 
