KEIMBAHN-DETERMINANTS 215 



become absorbed by active substances (Haecker, 

 1895, 1899) ; (4) nucleoli represent nutritive material 

 used by the nucleus into which it is taken from the 

 cytoplasm (Montgomery, 1899). 



Undoubtedly the various bodies known as nucleoli 

 originate in different ways, have different histories, 

 and perform different functions. 



In the particular cases to be discussed here the 

 nucleoli are not temporary structures, as is usually 

 true, but persist for a comparatively long interval after 

 the germinal vesicle breaks down. What seemed to 

 be the most important and convincing evidence of 

 the functioning of a nucleolus as a keimbahn-determi- 

 nant is that furnished by Silvestri (1906, 1908) in 

 parasitic Hymenoptera. As shown in Chapter V, 

 however, the "nucleolo" of Silvestri is really not a 

 nucleolus but consists of chromatin. 



As we have already noted, in a few instances the 

 nucleolus does not disappear during the maturation 

 divisions but persists for a time as a "metanucleolus" 

 (see p. 183). These metanucleoli are evidently of 

 a different nature from the usual type and are hence 

 saved from immediate disintegration in the cyto- 

 plasm. The localization of the metanucleolus in the 

 egg is the result of either its own activity, or that of 

 the surrounding cytoplasm, or a combination of these. 

 Gravity can have no decided effect upon it (Herrick, 

 1895), since its position is constant, whereas the posi- 

 tion of the egg with respect to gravity is not. It 

 also seems hardly possible that oxygenotactic stimuli 

 are the cause of its localization, as has been suggested 



