No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 443 



be more freely fluid. Further, at this period we usually find 

 vacuoles within some of the nucleoli of each germinal vesicle 

 (Figs. 217, 218, 226, 229-231); sometimes no vacuoles are 

 present in any of the nucleoli of a nucleus, but it is the rule that 

 at least one of them, and that usually the largest, contains one 

 or several vacuoles. Sometimes four or five of the nucleoli, 

 which may be very unequal in size, may each have vacuoles. 

 Occasionally a nucleolus contains only one vacuole, and in the 

 latter there may be one or several small solid bodies, which 

 stain like the ground substance of the nucleolus, and may be 

 termed nucleololi ; one of the latter may be fused with the inner 

 surface of the nucleolar ground substance (Figs. 217, 218, 230, 

 231). These nucleololi vary in number and size, and are 

 absent in the greater number of the vacuoles ; so no particular 

 significance should be attached to them, since they are probably 

 nothing more than portions of the ground substance of the 

 nucleolus which have become detached from the surrounding 

 substance and have come to lie within the vacuole. Durino- 

 this period the nuclear membrane is thinner than at any other 

 stage, and the nucleus is very noticeably amoeboid in form, the 

 amoeboid processes being much more pronounced than in any of 

 the other nemerteans examined ; these processes in reality repre- 

 sent changes in the form of the nucleus, and are not artefacts, 

 since they are seen equally well after preservation in the most 

 diverse fixing fluids (Figs. 226, 227, 230, 232, 233). The nu- 

 clear membrane is always particularly thin around these nuclear 

 processes, but, as far as I could make out, never becomes broken. 

 Third nucleolar stage. — The large nucleoli which were 

 present at the end of the preceding stage now commence to 

 fragment into smaller nucleoli, which are more or less equal in 

 size, and then the latter wander towards the periphery of the 

 nucleus ; at the conclusion of this period, which must take 

 place in a very short time, since I found only a few germinal 

 vesicles exhibiting it, there are a large number of rather small 

 nucleoli close to the nuclear membrane (Fig. 234). At this 

 time the nucleoli attain their maximum staining intensity ; the 

 nucleus usually shows no traces of an amoeboid form, and its 

 membrane has increased in thickness. None of the nucleoli 



