KEIMBAHN-DETERMINANTS 239 
soon as the primordial germ cells are established. 
The reason for this appears to be that localizations 
occur in holoblastic eggs at each cleavage and that 
not until the thirty-two-cell stage or thereabouts 
does the keimbahn material become entirely sep- 
arated from other organ-forming substances and 
segregated in a single cell. When this point is 
finally reached, this keimbahn material must neces- 
sarily become divided between the daughter cells. 
In practically all known cases the daughter cells 
of the primordial germ cells are equal in size and each 
receives an equal portion of the keimbahn-de- 
terminants (Fig. 37, B). This is certainly to be 
expected from their constitution and future history. 
Sagitta, however, differs in this respect, for the remains 
of the “‘besondere Kérper” appear to be unequally 
distributed between the two daughter cells of the 
primordial germ cells (Fig. 54) and both Elpatiewsky 
(1909, 1910) and Stevens (1910), therefore, consider 
this as probably a differential division whereby in this 
hermaphroditic animal the substance of the male 
primordial germ cell is separated from the female. 
More work is necessary to make certain of this point. 
Conciustion. Keimbahn-determinants are def- 
initely localized in the egg and in definite cleavage 
cells. This localization is first observable just 
before or just after the eggs are fertilized, or, in 
parthenogenetic forms, shortly before maturation 
and cleavage are inaugurated. Some mechanism in 
the egg must be responsible for this localization. 
Heterodynamic centrosomes may have some influence 
