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 Korper" 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. 



CONCLUSION. 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 



