226 GENERAL EMBRYOLOGY 



(6) to each other. Then second, we may emphasize chiefly 

 the physiological aspects of cleavage describing the relation of 

 the cleavage processes (a) to the structure or organization of 

 the ovum and zygote, (6) to the later stages in the development 

 of the mature organism. As a matter of fact these aspects of 

 cleavage are not really separate, for all the particular phenom- 

 ena of cleavage, as of development in general, are to be referred 

 to a single fundamental condition, namely, the organization 

 of the ovum or zygote as it is related to external conditions; and 

 if our knowledge were complete here we should be able to 

 describe all the phenomena of cleavage from a single viewpoint. 

 But for the present we shall find it more convenient as well as 

 more instructive to separate more or less arbitrarily and to 

 consider apart, the chiefly morphological and the chiefly physio- 

 logical aspects of this process. 



In considering the relation of cleavage to the grosser struc- 

 ture of the zygote we find that one of the primary factors in 

 determining the form of cleavage is the relative amount and the 

 form of distribution of the yolk and other deutoplasmic sub- 

 stances contained in the ovum. In Chapter III, three types of 

 eggs were described on this basis: (1) homolecithal or isolecithal 

 (alecithal), containing little deutoplasm, distributed with 

 considerable uniformity throughout nearly the entire ovum: 

 (2) telokcithal, containing varying, often considerable amounts 

 of deutoplasm chiefly localized toward the vegetative pole of 

 the ovum; (3) centrolecithal, really a form of telolecithal ova 

 in which the deutoplasm has a central rather than a polar 

 localization. 



Corresponding in a general way with these variations in yolk 

 distribution we may distinguish certain types of cleavage, each 

 however with certain variations which may sometimes appear 

 as connecting intergradations. First we may distinguish 

 complete and incomplete cleavage. When the eggs are com- 

 paratively small and of the homolecithal type, the earlier 

 cleavage planes pass completely through them in meridional 

 and latitudinal planes. Theoretically the simplest form of 

 complete cleavage is that known as equal cleavage, where the 



