THE BIOLOGY OF THE CELL SURFACE 



increases; (5) water Is redistributed; (6) the ectoplasm 

 increases. 



Before I begin the discussion of these events, I must deal 

 with the loss by an egg of capacity to produce more than 

 one embryo — i.e., with the change of the egg from a pluri- 

 potent to a unipotent system. This loss of plurlpotency, 

 often spoken of as a chief problem in development, in my 

 judgment, is only a revelation that embryogenesis is a series 

 of progressive restriction. 



Loss of pluripotency is revealed at that stage in cleavage 

 when the blastomeres having been experimentally separated 

 develop into defective embryos. In the eggs of snails, 

 for example, this loss can be demonstrated to have taken 

 place at first cleavage; blastomeres separated at this time 

 develop each to a swimming form, which is an incomplete 

 embryo made up of those structures which In the normal 

 embryo would have arisen from this blastomere. In eggs 

 of echinlds, on the other hand, loss of pluripotency occurs 

 later because only after the third cleavage do the blasto- 

 meres when separated develop into defective embryos; 

 blastomeres Isolated after first and second cleavage develop 

 into perfect though dwarf embryos, one-half and one-fourth, 

 respectively, the size of the normal. Briefly, we find that 

 animal eggs vary with respect to the time after fertilization 

 when they lose capacity for multiple embryo-production. 



Cleavage has been classified as determinate and inde- 

 terminate. In eggs with determinate cleavage, the history 

 of each cell or blastomere in a late period of the cleavage- 

 process can be traced back to the two cell stage; that is, 

 some eggs as early as the two cell stage reveal an organiza- 

 tion which fixes the destiny of the blastomeres derived 

 from the products of the first cleavage. In so-called inde- 

 terminate cleavage, although for a time the blastomeres 

 show constant origin, they later arise in undetermined 

 fashion. This classification of the cleavage of eggs as 



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