538 



ELEMENTARY PHYSIOLOGY 



LESS. 



4. The Differentiation of the Primitive Cells.— 

 Gradually, these undifferentiated cells become changed 

 into groups or sets of differentiated cells, the cells in one 

 set being like each other, but unlike those of other sets. 

 Each set of differentiated cells constitutes a "tissue," 

 and each tissue is variously distributed among the several 



Fio. 174. — The Si'ccessivk Division of the Mammalian Ovum into 

 Blastomeres. Somewhat diagrammatic. 



a, division into two, 6, into four, c, into eight, and d, into several 

 blastomeres. The clear ring seen in each case is the zonii pellucida, or 

 membrane investing the ovum. 



organs, each organ generally consisting of more than one 

 tissue. 



And this differentiation of structure is accompanied by 

 a change of properties. The undifferentiated cells are, 

 as far as we can see, alike in function and properties as 

 they are alike in structure. But coincident with their 

 differentiation into tissues, a division of labour takes 



