424 



ix//ekij:l\'ce axd developmext 



show that here the substance of the attraction-spheres is unequally 

 distributed, in a cjuito definite way, among the cleavage-cells, each 

 sphere of a daughter-cell being carried over bodily into one of the 

 granddaughter-cells (Fig. 192). We have here a substantial basis for 

 the conclusion that in cleavage of this tvpe qualitative division of the 

 cytoplasm may occur. 



It is important not to lose sight of the fact that development and 

 differentiation do not in any proper sense first begin with the cleavage 

 of the ovum, but long before this, during its ovarian history.^ The 

 primary differentiations thus established in the cytoplasm form the 

 immediate conditions to which the later development must conform ; 

 and the difference between AvipJiioxus on the one hand, and the 



Fig. 192. — Two successive stages in the third cleavage of the egg of Crepidula, seen from the 

 upper pole. [CONKLIN.] 



In both figures the old spheres (dotted) lie at the upper pole of the embryo, and at the third 

 cleavage they pass into the four respective cells of the first quartet of micromeres. The centro- 

 somes are seen in the new spheres. 



snail or ctenophore on the other, simply means, I think, that the 

 initial differentiation is less extensive or less firmly established in 

 the one than in the other. 



The origin of the cytoplasmic differentiations existing at the be- 

 ginning of cleavage has already been considered (p. l?>^). If the 

 conclusions there reached be placed beside the above, we reach the 

 following conception. The primary determining cause of develop- 

 ment lies in the nucleus, which operates by setting up a continuous 

 series of specific metabolic changes in the cytoplasm. This process 

 begins during ovarian growth, establishing the external form of the 

 egg, its primary polarity, and the distribution of substances within it. 

 The cytoplasmic differentiations thus set up form as it were a frame- 



1 See Wilson ('96), Driesch ('98, i). 



