CLEAVAGE AND DIFFERENTIATION 



For these three capital reasons, then, I consider it best 

 to limit my discussion of differentiation to the cleavage- 

 period. Since the fertilized uncleaved egg is itself dif- 

 ferentiated, and in this respect an unknown system, a 

 distinct advantage accrues in pushing back our inquiry as 

 far as possible to it as a single cell whence originate the many 

 different kinds of cells that make up the embryo. For the 

 great question is: how out of a single cell do these manifold 

 differences arise .^ I repeat: singleness does not mean sim- 

 plicity. And yet, inasmuch as the complex organism has 

 its genesis in a single protoplasmic system, the unknown 

 complexity of this system may in some measure become 

 known by means of a resolution of the events by which a 

 single cell organism becomes one of many cells. It follows 

 from this that we should define very exactly the cleavage- 

 period. I turn therefore to a brief resume of cleavage 

 processes in various animal eggs. Such a resume will 

 further be valuable, because in the later discussions I 

 shall need to refer to the various cleavage-types; and 

 because by evaluating differences in the mode of cleavage 

 we may arrive at a basis of similarity. 



Every developing animal egg normally passes through a 

 period of successive cell-divisions or cleavages. The 

 cleavage-cells or blastomeres always arise by binary fis- 

 sion, that is, two "daughter-cells" arise from the division 

 of one cell. In this fashion is established a cleavage-pattern 

 which differs with different eggs. Upon these differences 

 depends the classification of eggs according to their type of 

 cleavage. Cleavages are classified as total and partial: 

 total, if the whole egg cleaves; partial, if only a part of the 

 egg cleaves — this cleaving portion is always either the whole 

 surface or a definite surface-area. 



In total cleavage the blastomeres of first cleavage and 

 of several subsequent divisions may be approximately equal 



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