THE BIOLOGY OF THE CELL SURFACE 



periods. Development is a catenary process of overlapping 

 stages and can not be categorically separated by rigid lines. 

 With periods three and four, which have to do with the 

 establishment of the organs and the finer differentiation 

 within them, we have here no concern. Period two includes 

 the formation of the primary sheets of cells, the ectoderm 

 and the endoderm in the lowest multicellular animals, the 

 sponges and the coelenterates, and the ectoderm, endoderm 

 and mesoderm in all the other multicellular animals. 

 Period two represents the end-result of period one. It is 

 in this first period, cleavage, that our problem lies. And 

 this for the following reasons: 



Coming first, this differentiation during cleavage stands 

 nearest to the condition in the egg at the moment of fer- 

 tilization, a moment that forms in time the limit between 

 the condition of the egg as single cell and those conditions 

 of the egg as multiplying cells which we call the differentia- 

 tion during cleavage. Hence studying the period of cleav- 

 age, we approach the source whence emerge the progres- 

 sively branched streams of differentiation that end finally 

 in almost quiet pools, the individual cells of the complex 

 adult organism. 



But just as the source of a river though single may not 

 be simple but compounded of rivulets, so the fertilized egg 

 though single is not simple, being itself a complex of many 

 contributary streams of differentiation. For the fertilized 

 egg is already a differentiated system. Indeed, the whole 

 history of the egg from the moment when it became dis- 

 tinguished as such and separated from body cells is a suc- 

 cession of diiTerentiations. In form, growth, mode of 

 building up of food reserves, and in nuclear structure, a 

 young egg-cell shows itself different from every and any 

 other cell of the animal's body. The egg-cell is subjected 

 to the same environmental influences as other, fully special- 

 ized cells of the animal's body. The changes and processes 



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