DEVELOPMENT OF THE FERTILIZED EGG 283 



2. Differentiation. Although the cells at the outset are 

 much alike, they soon begin to show differentiation. In Fig- 

 ure 132 B it will be seen that the upper cells are smaller 

 than the lower ones, and the contents of the larger cells 

 are quite different from those of the smaller. The difference 

 thus shown early in the development of the egg marks the 

 distinction between those cells which will eventually form the 

 alimentary canal and those which will form the other parts 

 of the body. As the development goes on and the number 

 of cells in the embryo increases more and more, greater and 

 greater differences are found among them (Figs. C and D), 

 so that one group of cells after another becomes set apart by 

 differences in structure and functions, until finally, when the 

 animal has reached the adult form, it is not only composed of 

 innumerable cells, but these cells have assumed a great variety 

 of shape and function. This process of gradual change of 

 shape and function of cells which were originally alike, is spoken 

 of under the name of differentiation. A similar change occurs 

 in all multicellular animals and plants; for, after segmentation 

 of the egg, there always follows a differentiation of cells. 



3. The Formation of Germ Layers. After the cells have 

 multiplied until they have become quite numerous, they begin 

 to arrange themselves in three groups. Soon there appears an 

 outer layer, an inner layer, and a middle layer, known respec- 

 tively as ectoderm, endoderm, and mesoderm. These are 

 shown in Figure 132 Z), which represents a later development 

 in the frog. The method by which these three layers are formed 

 is shown diagrammatically in Figure C. It may briefly be 

 described as the growing of the mass of the smaller, ectoderm 

 cells, around and over the larger, endoderm cells, so as finally 

 to bring the larger cells upon the inside of the embryo, surrounded 

 by the smaller ones. Meantime there has grown from the 

 outer and inner layers a third mass of cells, the mesoderm, 

 that pushes its way between the other two, thus partly filling 

 up the space between the outer and inner layers; Fig. D. The 



