246 GENERAL EMBRYOLOGY 



the time at which the organization of the egg becomes suffi- 

 ciently complete to be effective in determining the course of 

 differentiation of the blastomeres, may vary. Thus cleavage 

 would be completely determinate if the egg organization were 

 entirely or largely completed before the cleavage process begins, 

 incompletely determinate if the organization is only partial, 

 and indeterminate if the organization is only slightly 

 marked during the earlier cleavages. For egg organization is 

 progressive, it develops. So the determinate or indeterminate 

 character of cleavage may depend, partly at least, upon the 

 relative time during cleavage at which the organization becomes 

 marked to such an extent as to determine the fate of particular 

 blastomeres. Other factors obviously enter into the process 

 and we shall review the subject from another point of view in 

 the next chapter. 



We have seen above that in nearly all species the earliest 

 cleavage planes are definitely related to the polar axis of the 

 ovum. The polarity of the egg is one of the fundamental 

 aspects of its organization. We have seen also 'that the ovum 

 often contains formed substances of various kinds, both proto- 

 plasmic and deutoplasmic, distributed in the cytoplasm in a 

 definite and usually specific manner/ It is a common feature 

 of cleavage that the first plane symmetrically divides the egg 

 or that part of it which takes part in the process of cleavage. 

 And furthermore, with very few exceptions, this first cleavage 

 plane coincides either precisely or approximately with the 

 median plane of the embryo and adult. Nereis is one of the 

 few exceptions to this rule; in this Annelid, as in some of the 

 Urodeles, the second plane marks the future median plane. 



The factors which appear-immediately to determine the loca- 

 tion of the first cleavage plane are mainly two. First and most 

 important is the structure of the ovum itself, which in many 

 cases, even in the unfertilized condition, is obviously bilaterally 

 symmetrical; the first cleavage plane corresponds closely with 

 this plane of symmetry, which is therefore determined by the 

 same " organizational " factor that determines the polarity and 

 other structural features of the ovum. In other cases the 



