CHAPTER LS 



Ontogeny in Ciliatcs 

 and the Diflferentiation 

 of Metazoan Cells 



Morphogenesis of an animal may be considered the result 

 of cell differentiation. Only the germ line remains toti- 

 potent. The other cells become specialized, and develop- 

 ment results from the interaction of differentiated cells and 

 from the actions and interaction of the products of specific 

 activities of differentiated cells. An organism is the equili- 

 brated sum of differentiated cells. One of the problems 

 of development is the problem of the nature and origin of 

 cell diversification and specialization. 



We know that the genome as a whole is a principle of 

 stability and that genes control enzymes. But development 

 is the story of diversification of cells possessing the very 

 same genome. It is convenient to visualize diversification 

 as the result of segregation of specific cytoplasmic particles, 

 which, with the collaboration of genes, would be responsible 

 for the synthesis of enzymes or group of enzymes. It is 

 possible also with P. Weiss (1949) to consider differentia- 

 tion as "si chain of events consisting of alternate physical 

 regrouping and chemical alteration of the molecular popula- 

 tions, the latter phenomenon involving the emergence of 

 novel species of compounds." Whatever our opinions on 

 differentiation may be, it happens that in many eggs deuto- 

 plasmic reserves of different types are unequally distributed. 

 Cleavage necessarily separates blastomeres possessing dif- 

 ferent reserves, that is to say, different types of metabolism. 



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