CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



93 



time of maturation and fertilization, the 

 new arrangement also is a function of its 

 molecular constitution and has a definite 

 topographic relation to the form of the 

 future organism. 



Since cellular differentiation takes place 

 in the cytoplasm, we are concerned here 

 more directly with this constituent of the 

 cell, bearing in mind, however, that the 

 cytoplasm is accompanied and ultimately 

 controlled by the genie complex in the 

 chromosomes. Since the latter is presum- 

 ably the same for all cells of the organism, 

 differences between cells must arise through 

 interaction between the constant genom and 

 the locally variable cytoplasm, in which 

 they ultimately become visible. 



In the sense that the organism is made up 

 of the same specific protoplasm as that oc- 

 curring in the egg, together with its prod- 

 ucts or differentiations, the characters of 

 the organism are predetermined in the 

 germ. The actual realization of these char- 

 acters, however, comes about during de- 

 velopment in a variety of ways. In the 

 so-called mosaic eggs, i.e., those with ''pre- 

 cocious segregation," to use Lankester's 

 (1877) term, local differentiation arises be- 

 fore cleavage and a minimum of modifica- 

 tion in response to internal environmental 

 factors is involved in subsequent develop- 

 ment. In other eggs the protoplasm long 

 remains labile with respect to its prospec- 

 tive differentiation, and in these many or- 

 gans of the embryo develop only through 

 the interaction between the cells producing 

 them and other elements in the internal en- 

 vironment, i.e., other cells and the internal 

 medium. In such eggs local predetermina- 

 tion is reduced to a minimum. Their mem- 

 bers in continuance are fashioned, when as 

 yet there is none of them. 



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