METAZOAN ORGANIZATION 117 



divisions extend completely through the zygote, forming nearly 

 equal cells. If the yolk is concentrated in one end of the egg, the 

 divisions of the developing embryo are unequal. During the early 

 divisions all of the cells of the body divide at so nearly the same 

 rate that it appears as if the zygote were being cut with a knife or 

 cleaver into smaller parts. This process provides for the rapid in- 

 crease in the number of cells and growth of the embryo which is 

 necessary before any special parts can be formed. Cleavage will 

 be described more fully in a later chapter under the discussion of 

 the development of the frog. 



As divisions proceed, a hlastula is formed by the development of 

 a cavity (blastocoele) within the spherical mass of cells, the wall 

 of which is now a single layer. The formation of the blastula, which 

 usually comes at the sixty-four cell stage or later, marks the end 

 of cleavage. The blastula stage of an animal like a starfish or a 

 frog resembles somewhat a hollow rubber ball whose wall is made 

 up of a large number of pieces cemented together. 



As cell divisions continue in the blastula, a gastrula is finally 

 formed. The blastula does not simply increase in circumference, 

 but there comes a time when the wall on one side pushes in (in- 

 vaginates), finally meeting the wall of cells from the other side. 

 This gradually crowds out the cavity and forms a wall of two layers 

 of cells. The outer layer is known as the ectoderm (outer skin) and 

 represents the portion of the wall of the blastula which has not 

 folded in. The inner layer, or that resulting from the infolding of 

 the wall of the blastula, is called endoderm (inner skin). As divi- 

 sion of cells in this wall proceeds and the infolding continues, the 

 two margins of the infolded part come nearer and nearer each other. 

 This gradually encloses an outside space which is lined by the 

 endoderm and represents the primitive digestive tract or archenteron. 

 This is the beginning of the two primitive germ layers, ectoderm and 

 endoderm. In sponges and coelenterates development stops here. 



In higher forms, immediately following gastrulation, a third germ 

 layer, the mesoderm (middle skin), is organized from cells usually 

 contributed by one or the other or both of the other germ layers. 

 In some cases it arises as two saclike outgrowths from the endoderm, 

 one on each side in the gastrula. These pouches push into the 

 remains of the blastocoele. In other cases separate cells are shed 

 from ectoderm or endoderm or both, or from an undifferentiated 



