No. 2.] COMPARATIVE CYIOLOGICAL STUDIES. 42 1 



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.), 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 



