GENERAL ZOOLOGY 



Fig. 5.23. The principal localizations of cells and their diflferentiation during the development 

 of the vertebrates. 



shape associated with locaHzation of specific groups of cells result from in- 

 trinsic capacities of the cells themselves. This has been shown by using 

 enzymes to digest away the intercellular material which binds the cells of the 

 early embryo together. Such dissociated cells will, under carefully controlled 

 conditions, undergo the changes of shape and the movements characteristic of 

 similar cells within the embryo. 



Cell Differentiation. During the various movements of masses of cells 

 in relation to their neighbors that occur as localization takes place, no 

 essential changes in the character of the cells can be noted. When a group 

 of cells has finally shifted its position, however, cell differentiation, or histo- 

 genesis, begins; that is, the cytosomal changes that give rise to the tissues 

 of the adult occur. The differentiation or specialization of the somatic cells 

 will not be described in detail. By referring to Figure 5.23 you can correlate 

 the localizations and differentiation of development with the anatomy and 

 histology of the adult vertebrate. Differentiation of certain kinds of somatic 

 cells is not confined to the developmental period. Cells wear out and are re- 



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