156 BIOLOGICAL LECTURES. 



nuclear history, as is shown very clearly in the germinal vesicles 

 of Nemerteans, where the nucleoli are formed at the periphery, 

 wander towards the centre, and finally pass to the periphery again. 

 Similar "migrations" have been described also for Amphibia. 



Within nucleoli vacuoles are usually present, which are 

 probably, in most cases at least, fluid globules ; they are nor- 

 mal structures, since they may frequently be seen in the liv- 

 ing cell. The origin of these vacuoles or globules was traced 

 by me in the germinal vesicles of nudibranch mollusks. In 

 these objects the nucleolus is at first homogeneous, but with 

 its increase in size, which keeps pace with that of the nu- 

 cleus, lighter staining globules make their appearance within 

 the nucleolus. Just at this time, fluid drops of similar chem- 

 ical nature are seen within the caryolymph, and these drops, on 

 the other hand, resemble chemically and structurally the early 

 yolk globules, which are simultaneously appearing in the cyto- 

 plasm. Consequently, we may conclude that globules which 

 are probably closely related to the earliest yolk globules are 

 taken up into the nucleus, probably as nourishment, and that 

 some of them are deposited within the nucleolus in the form of 

 intranucleolar globules or vacuoles. Thus, at least in these 

 objects, there are facts to show that the nucleolar vacuoles are 

 extranucleolar in origin. In these ovarial ova the vacuoles 

 gradually fuse together until a single large, excentric vacuole 

 is formed, bounded by a thin layer of nucleolar ground sub- 

 stance. Similar fusions of vacuoles to produce one large one 

 are found also in nucleoli of many other germinal vesicles, as, 

 e.g., in Polydora and Piscicola. In the objects studied by me 

 the nucleolar vacuoles would appear to be in all cases fluid 

 globules and not empty spaces, which is proved by their be- 

 coming stained with suitable reagents. In some cases these 

 vacuoles are not spherical (the usual case), but irregular in out- 

 line ; and the latter form may be easily explained by the as- 

 sumption that their consistency is occasionally viscid. The 

 same assumption serves to explain likewise the irregular forms 

 of many nucleoli. That is to say, the form of the nucleolus 

 and of 'its contained vacuoles will be spherical, only provided 

 that they have a thinly fluid consistency. 



