CLEAVAGE AND DIFFERENTIATION 



In discoidal cleavage the cleaving disc is located at this 

 tgg pole. In superficial cleavage, the initial nuclear divi- 

 sions are confined to the region directly below the point 

 whence the polar bodies form, whilst the cleavage itself 

 involves the whole egg-surface. 



Because hitherto some writers have preferred to assume 

 a simplicity for the uncleaved egg, thus disregarding its 

 previous history as a succession of differentiations, fruit- 

 less attempts have been made to interpret the cleavage- 

 patterns of the animal &gg^ diverse though they are, as the 

 mode for or even the cause of the setting-up of those dif- 

 ferences whose sum-total defines the differentiation made 

 manifest during cleavage. It is, however, more cor- 

 rect to see in the cleavage-pattern a display of the differ- 

 entiations present before cleavage set in. The separa- 

 tion of the eg^ as egg from other cells of the organism, 

 its position in the ovary, its relation to food-supply, its 

 intake and elaboration of food both in quantity and quality 

 — in short all those processes that long before fertiliza- 

 tion and cleavage endowed the eggs with those character- 

 istics to which many authors relate cleavage as total or 

 partial and as variants of each of these — these are the proc- 

 esses which condition the respective cleavage-patterns. 

 Thus, a cleavage-pattern is established by earlier occurring 

 differentiations; It does not express differentiation occurring 

 during cleavage. We therefore consider the cleavage- 

 pattern as only the frame-work within which we try to 

 find the answer to our question. By analysis of the events 

 which occur during cleavage, we may reach an agreement 

 upon a proposition as to the primary and basic factor which 

 underlies all forms of differentiation. For the cleavage 

 these chief events are six. 



(i) The egg is split up into cells; (2) the embryonic axis 

 and the plane of bilateral symmetry are revealed; (3) the 

 cytoplasmic inclusions shift positions; (4) nuclear material 



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