110 AN INTRODUCTION TO ZOOLOGY 



attached to one another as was noted in the case of Pandorina 

 (p. 94, Fig. 45, II). The resemblance of the group of blas- 

 tomeres to a mulberry suggested the term morula, which is often 

 used in describing the egg, during the early cleavage stages. 



Blastula. As cleavage advances, a cavity becomes noticeable 

 in the center of the egg (Fig. 53, H), becoming larger as develop- 

 ment proceeds until the whole resembles a hollow rubber ball, 

 the rubber being represented by a single layer of cells. At this 

 stage the egg is called a blastula, the cavity the cleavage or seg- 

 mentation cavity, and the cellular layer the blastoderm, The 

 blastula resembles somewhat a single colony of Volvox (Fig. 

 46, A). 



Gastrula. The cells on one side of the blastula are seen to be 

 thicker than elsewhere (Fig. 53, K) and begin to invaginate 

 (Fig. 53, L). This process results in a cup-shaped structure with 

 a wall of two layers, an outer layer of small cells and an inner 

 layer of larger cells. The embryo may now be called a gastrula 

 (M), and the process by wHich it developed from the blastula 

 is termed gastrulation. The cleavage cavity is almost obliterated 

 during the invagination, while a new cavity, the primitive diges- 

 tive tract or archenteron, is established. 



Germ layers. The cells of one layer of the gastrula resemble 

 one another, but differ in appearance from the cells of the other 

 layer. Each layer gives rise to certain definite parts of the body, 

 and is therefore termed a germ layer; the outer is the ectoderm 

 (Fig. 53, N, ak.), the inner, the entoderm (N, ik.). Animals with 

 only these two layers are said to be diploblastic; but the majority 

 of the higher animals have a third layer, which usually appears 

 between the first two after the gastrula has been formed. This is 

 the middle layer, or mesoderm. It originates either from the pro- 

 liferation of a few special cells which may be recognized in the 

 early cleavage stages, or from cells budded off from the inner 

 surface of both the ectoderm and entoderm, or from pouches 

 arising from the walls of the entoderm (Fig. 53, N). Animals 

 with three germ layers are said to be triploblastic. 



