DEVELOPMENT AND DIFFERENTIATION OF CELLS. 25 



The embryonic body or erribryo of every higher plant and ani- 

 mal is derived from the germ-cell by a process essentially like that 

 just described, though both the form of the cells and the order of 

 division are usually more or less irregular. In animals the cells 





PIG. 14. Cleavage or segmentation of an ovum, showing successive division of the 

 germ-cell (a) into two (ft), four (c), and eight ((?) Later stages are shown ate 

 and /. The first four figures are diagrammatic ; e and / are after Hatschek's fig- 

 ures of the development of a very simple vertebrate (AmpJiioxus). 



thus formed are usually naked at first, though they often ac- 

 quire a membrane in later stages. Among plants, on the con- 

 trary, the cells usually possess membranes from the first, prob- 

 ably because their need for a firm outer support is greater than 

 the need for free movement demanded bv animals.* 



/ 



Modification of the Embryonic Cells. Differentiation. The 

 close similarity of the embryonic cells does not long persist. As 

 development proceeds, the cells continually increasing in number 

 by division become modified in different ways, or differentiated, 

 to fit them for the many different kinds of work which they have 

 to do. Those which are to become muscle- cells gradually assume 

 an entirely different form and structure from those which are to 

 become skin-cells ; and the future nerve- or gland-cells take 

 on still other forms and structures. The embryonic cells are 

 gradually converted into the elements of the different- tissues- 

 this process beino; the differentiation of the ti^in/s which has 



i ~ ie' / 



* For a more precise account of cell-division see p. 83. 



