CLEAVAGE 245 



We have already seen that the exceptions to these rules are so 

 numerous and so fundamental that they must have some real 

 significance. It is now clear that in determinate cleavage all 

 the details have a significance that is prospective, looking toward 

 the structural and physiological characteristics of the larva or 

 fully formed organism. It has been said regarding determinate 



cleavage that "One can go over every detail of 



cleavage, and knowing the fate of the cells, can explain all the 

 irregularities and peculiarities exhibited" (Lillie). 



Why this should be true is partly explained when we remem- 

 ber that the characters of cleavage and of the fully developed 

 organism are both the primary result of the underlying structure 

 of the ovum. Cleavage stands as an intermediate process 

 between egg organization and adult structure; it is one of the 

 processes through which the primary organization of the ovum 

 gains expression in adult form. 



This view of the cleavage process is by no means the only, 

 or the original view, but it serves to bring out clearly the fact 

 that the problems as to the nature and causes of the differ- 

 entiations occurring during the cleavage process are related to 

 the problems of the nature and causes of the differentiations of 

 adult structure. Indeed these differentiations have a common 

 cause in the structure and reactions of the ovum, and are there- 

 fore fundamentally equivalent. In our introductory chapter 

 we said that the organism is specific at every stage, the zygote, 

 the group of blast omeres, the embryo, the adult, are all the 

 same specific organism, and the question why the cleavage 

 group is what it is, is the same as the question why the mature 

 organism has its own specific and individual characteristics. 

 The problems and processes of development are fundamentally 

 alike throughout. 



In continuing our discussion of this determinative aspect of 

 cleavage we shall make little further attempt to distinguish the 

 determinate and indeterminate forms since this separation is 

 clearly artificial. The apparent differences between deter- 

 minate and indeterminate cleavage may arise from the fact 

 that one of the determining factors contains a variable. That is, 



