244 GENERAL EMBRYOLOGY 



development is a process, not a succession of morphological 

 stages, and there remains to be described the most important 

 aspect of cleavage as a developmental process. We come then 

 to still another classification of cleavage types as (1) deter- 

 minate and (2) indeterminate. 



Cleavage is said to be determinate when exact morphological 

 and physiological relations exist between the individual blasto- 

 meres and, (a) specific structures in the embryo and fully 

 developed organism, and also (b) specific regions or substances 

 in the ovum. Each of the products of cleavage is here a true 

 organ, of particular and known value in development: blasto- 

 meres are not interchangeable, and their removal or destruction 

 may lead to specific, related defects or abnormalities in later 

 development (e.g., the Ascidians). Cleavage is described as 

 indeterminate when the blastomeres seem to have no specific 

 relation to the structure of either the egg or embryo and adult. 

 Here all the blastomeres may have equal value as factors in 

 development; they are more or less interchangeable, and 

 removal or destruction leads only to the loss of a corresponding 

 amount of substance, not to the absence of any specific or 

 related parts (e.g., the Echinoderms). 



In order that this classification should not be misleading, it 

 should be said at once that this is a purely artificial distinction, 

 based rather upon historical grounds than upon the facts of 

 development, for these two types are completely connected by 

 transitional conditions, and soon or late in development, all 

 cells come to have specific, determined values. 



Such a grouping as this, of the varieties of cleavage, obviously 

 rests primarily upon a physiological rather than a morphological 

 basis, for cleavage here is regarded as a process of development. 

 In all cases of determinate cleavage the essential fact is that 

 cleavage is not the mere division of the zygote into separate 

 masses and units which can be moved about and moulded into 

 the form of an embryo; cleavage is not merely a series of cell 

 divisions, not the mere " vegetative reduplication of parts" 

 occurring in accordance with certain mechanical rules like 

 those of Balfour, Sachs-Hertwig, or Plateau, mentioned above. 



