CLEAVAGE AND DIFFERENTIATION 



by which it becomes an embryo thus spring from an initial 

 differentiation, an intrinsic organization of the egg that 

 distinguishes it from all other cells. 



Consider the chick at the moment when it emerges from 

 the egg-shell: it has all the organs that it will ever have 

 and it had the precursors of these at the moment that it 

 was fertilized. Nothing of organic material did it gain 

 during the 21 days of incubation. It took in water, oxygen 

 and heat. But not one single milligram of living proto- 

 plasm is in the hatched animal but what was within the 

 unincubated egg. Early the embryo formed blood with 

 a red pigment; this pigment contains iron. But not the 

 most minute fraction of this iron was gained during devel- 

 opment; all present in its body at hatching was already 

 there before the egg was laid. 



True, a mammalian embryo, like that of the human, 

 develops by the material of the blood which comes to it. 

 But in the minute human Qgg must be present all that from 

 which comes a human embryo for the blood which nourishes 

 it is the same as that which supplies any other cell in the 

 mother's bod}', no one of which becomes a human embryo. 

 Hence, though the human egg differs from the bird's, at 

 basis the same is true: the egg at beginning of development 

 is a differentiated and a differentiation-capable system. 



Following its period of primary differentiation the egg 

 cell grows, reaching a stage prepared for fertilization but 

 unfertilizable. Then it becomes fertilizable — another phase 

 of differentiation. If fertilization ensues, it becomes dif- 

 ferentiated again. Thus differentiation flows on as a stream 

 without breaks and if later for purposes of convenience I 

 use words that seem to connote sharply discontinuous 

 phases, the reader should keep in mind that I regard these 

 phases, periods or stages, only as moments in time when 

 the stream alters its course. Although we can not yet 

 assert what in all this history is cause and what effect, 



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