DEVELOPMENT 275 



326. Differentiation of cells. When there are two cells 

 (Fig. in, &), they are exactly alike. When each of these 

 divides, the four resulting cells are exactly alike. By repeated 

 divisions the number of cells is rapidly increased. In some 

 species the cells are all alike until a very large number are 

 present. In other species of animals it is possible to observe 

 a difference of size after only a few divisions (see Fig. 112). 



FIG. 112. Early stages in the development of a frog 



In the frog's egg there is a considerable amount of food matter, or yolk, in addition to 

 the protoplasm. This yolk material is heavier than the protoplasm, and remains at the 

 bottom of the mass. So long as the cell divisions are in a vertical plane, b, c, all the 

 cells formed may be alike ; but when walls are formed in a horizontal plane, d, the upper 

 cells will be smaller than the lower ones, for while all the cells may have the same 

 amount of protoplasm, the lower ones will have larger quantities of yolk and will thus 



be larger, e,f,g 



At first all the cells divide at about the same rate. Later 

 the cells in one region divide more rapidly than those in 

 other regions. 



We can readily understand that while a starved cell may not be 

 able to do as much as a well-nourished cell, a cell that contains a large 

 surplus of inert food material may yet not be as active as one that is 

 not thus handicapped. In Fig. 112 the cells on the upper surface are 

 not only smaller, but in a given time they will also become more 

 numerous, because the cell division proceeds at a more rapid rate. 



With inequalities in the rate of division, and inequalities in 

 the sizes of the cells, the shape of the whole mass soon 

 departs from that of a perfect sphere (Fig. in). With the 



