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. 



Fourtli 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. Zygoiiemertes 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 



