CHAPTER 1 



Inlrocliictioii of tlir Kinrtosome 

 Consi(i(*rc(l as a Model 

 of a Visible Cytoplasmic Unit 

 Endowed with Genetic Continuity 



Development of an animal is a complex process generally 

 considered as the result of interactions between a specific 

 protoplasm and hereditary outfit on the one hand, and a 

 complex set of environmental factors on the other. The 

 most important phenomena during this process are egg 

 cleavage and subsequent cell multiplication, cell move- 

 ments, differentiation, and modulation, the sum of which 

 results in the formation of tissues, organs, and organism. 

 Development is an epigenetic phenomenon. Starting from 

 an egg, which is a highly complicated cell, every organism 

 has to be elaborated at each generation. This is ontogenesis. 



During development, different cell lines acquire different 

 properties. Differentiation is the process resulting in spe- 

 cialization of a cell as evidenced by its distinctive actual and 

 potential functions. It is by definition irreversible. 



It is known that there is, in general, a marked antagonism 

 between cell differentiation and cell division, that some 

 highly specialized cells are unable to divide, that the devel- 

 opment of an animal from an egg is, as a whole, an irrevers- 

 ible process, that evolution itself is irreversible. It is known 

 also that the animal has, with a few exceptions, relegated 

 to the germinal hne the responsibility for the maintenance 



of the species. 



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