REPRODUCTION AND DEVELOPMENT 265 



FERENTiATioN. {b) Cell clivisions, mechanical foldings of the cell 

 layers, and the migration of cells are in normal development care- 

 fully and accurately adjusted to the time of difTerentiation of the 

 several types of cells, so that development is an orderly process. 

 Cell divisions and mechanical changes in position of the ceils, and 

 differentiation are, however, independent processes. Experimentally 

 one may interfere with, and prevent cell divisions without arresting 

 the chemical changes of differentiation. With these facts of the 

 normal concurrence of differentiation and changes in position of 

 the cells and cell layers in mind, the description to follow will be 

 largely confined to descrip- 

 tions of change in form dur- 

 ing development. 



Cleavage. The first pe- 

 riod of development is marked f~r'^?tfVi!^^B5T ^^^^^^^^^'-^ 

 by a series of cell divisions 

 and is known as the cleavage 

 period. Division into two, 



then into four, then into eight Fig. 178. — The blastula stage of the cepha- 

 - ,, , . lochordate, Amphioxus. 



and mere ceils, results m a 



hollow sphere of cells known as the blastula (Fig. 178). Essentially 

 this process takes place in all forms of animals, but with consider- 

 able modification in some. The cavity of the blastula is called the 

 BLASToccELE. The truly spherical blastula of the lower invertebrates 

 resembles the general structure of the colonial protozoon, volvox, 

 which was described elsewhere (p. 89; p. 90). Biologists permit 

 themselves to speculate as to whether this resemblance indicates that 

 the ancestor of the Metazoa was a colonial protozoon of the struc- 

 tural type of volvox, or whether the resemblance is due to the 

 possibility that this sort of hollow sphere is a necessary stage in the 

 development of an adult animal regardless of its ultimate ancestry. 

 Gastrulation. The blastula is a single layer; the next step 

 in development is the formation of a diploblastic form from the 



