464 ROBERT W. HEGNER 



either of its own activity, or of that of the surrounding cytoplasm, 

 or a combination of these. Gravity can have no decided effect 

 upon it (Herrick, '95) since its position is constant, whereas the 

 posterior end of the egg with respect to gravity is not. It also 

 seems hardly possible that oxygenotactic stimuli are the cause 

 of its change of position as has been suggested by Herbst ('94, 

 '95), for the migration of the blastoderm-forming cells from the 

 center to the surface of the eggs of certain arthropods. Con- 

 cerning the fate of this nucleolus, one of the most difficult phenom- 

 ena to explain is its fragmentation at a definite developmental 

 stage and the apparently equal distribution of its substance to 

 the primordial germ cells during their multiplication. 



Haecker ('97) has suggested that the 'Aussenkornchen' which 

 appear in the egg of Cyclops during the formation of the first 

 cleavage spindle may be nucleolar in nature. Later ('03) this 

 idea was withdraw^n and more recently Amma ('11) has likewise 

 been unable to sustain this hypothesis. The most convincing 

 data furnished by Amma are that in an allied form, Diaptomus 

 coeruleus (fig. 17, H), these granules appear before the cleavage 

 spindle is formed and before the nucleoli of the pronuclei have 

 disappeared. 



The remaining forms in which nucleoli have been considered 

 as Keimbahn-determinants are merely suggestive. In Aequorea, 

 Haecker ('92) traced the metanucleolus, which arises from the 

 germinal vesicle, into certain cells of the blastula. Similar bodies 

 appear in Mitrocoma (Metschnikoff, '86), Tiara (Boveri, '90), 

 Stephanophyes (Chun, '91), Myzostoma (Wheeler, '97), and 

 Asterias (Hartmann, '02), but their fate has not been determined. 



It seems probable that in all these cases the same influences 

 may be at work regulating the time, the place, and the method 

 of localization of the nucleoli. Silvestri has made no attempt to 

 explain this behavior. The writer can only conclude (1) that 

 the 'nucleolo' of Silvestri and the 'metanucleoli' of other authors 

 differ in nature from ordinary plasmosomes, chromatin-nucleoli, 

 and double-nucleoli; (2) that these bodies are definitely segre- 

 gated in a certain part of the egg or in a certain blastomere, prob- 

 ably by protoplasmic movements; and (3) that their disinte- 



