ORIGIN OF CELLS. 53 



us the model after which all others are built, in spite of endless 

 variations in detail ; for the typical germ-cell gives rise to all 

 other cells in the body. This process is as follows : 



The first step of development (Fig. 21) consists in the division 

 of the egg into two similar halves, which differ from the original 

 cell only in lacking membranes, both being surrounded by the 

 membrane of the original cell. Each of the halves divides into 

 two, making four in all; these again into two, making eight,, 

 and so on throughout the earlier part of the development. By 

 this process (known as the cleavage or segmentation of the 

 egg) the germ-cell gives rise successively to 2, 4, 8, 16, 32, 64 r 

 etc., descendants, forming a mass of nearly similar cells, out of 

 which, by still further division and growth, the body of the 

 future animal is to be built up. These cells are only slightly 

 modified, but differ in most animals from the typical germ-cell 

 in having at first no surrounding membranes. The membrane of 

 the original germ-cell meanwhile disappears. 



The embryos of all higher plants and animals are 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 is 

 usually more or less irregular. In animals the cells thus formed 

 are usually naked at first, though they often acquire membranes 

 in later stages. Among plants, on the contrary, the cells usually 

 possess membranes from the first, probably because their need 

 for a firm outer support is greater than the need for free move- 

 ment demanded bv animal cells. 



*j 



Modification of the Cell. Differentiation. The close similar- 

 ity of the embryonic cells does not long persist. As develop- 

 ment 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 grad- 

 ually converted into the elements of the different tissues this 

 process being the differentiation of the tissues which has al- 

 ready been mentioned on p. 15 and are in this way enabled to* 

 effect a physiological division of labor. 



The variations in form and structure which thus appear are 



