KEIMBAHN-DETERMINANTS Q15 
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 fora 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 
