THE PROCESS OF CLEAVAGE. 57 



There are some important deviations from the process of division 

 just described, which affect the form of the cleavage products, although 

 leaving unaltered the finer processes relating to the nucleus. The 

 deviations are induced, as we shall show more in detail in the in- 

 dividual cases, by the variation in the amount of deutoplasm contained 

 in the eggs, and by the previously described variability in its distribu- 

 tion. One may appropriately separate the various forms of the 

 process of cleavage into two classes, and each class into two sub- 

 classes, although the forms merge into one another by means of 

 transitional conditions. 



To the first class we assign such eggs as are' completely divided 

 into segments by the process of cleavage. The cleavage itself we 

 designate as total ; and according as the segments are of equal or un- 

 equal size, we distinguish as subdivisions equal cleavage and unequal 

 cleavage. 



With total is contrasted partial cleavage. This occurs in the 

 case of eggs which are provided with very abundant deutoplasm, 

 and are consequently of considerable size, and in which, at the same 

 time, the previously described separation into formative yolk and 

 nutritive yolk has been distinctly established. In this case the for- 

 mative yolk alone undergoes a process of cleavage, whereas the chief 

 mass of the egg, the nutritive yolk, remains undivided, and in general 

 unaffected, by the processes of embryonic development ; hence the 

 name partial cleavage. This, in turn, is resolvable into the two sub- 

 types of discoidal and superficial cleavage, according as the forma- 

 tive yolk rests as a disc upon the nutritive yolk, or envelops the 

 latter as a thick cortical layer. KEMAK has designated eggs with 

 total segmentation as holoblastic, those with partial segmentation as 

 meroblastic. 



We may therefore present the following scheme of cleavage : 



I. TYPE 



Total cleavage : ^ 



(#) Equal cleavage Holoblastic eggs. 



(b) Unequal cleavage J 

 II. TYPE 



Partial 'cleavage : ~\ 



O) Discoidal cleavage Meroblastic eggs. 



(i) Superficial cleavage J 



I a - Equal Cleavage. 



In the general consideration of the process of cleavage we have 

 already become acquainted with the phenomena of equal segmenta- 



