AN INTRODUCTION- TO THE METAZOA 89 



blastula is termed gastrulation. The cleavage cavity is almost 

 obliterated during the invagination, while a new cavity, the 

 primitive digestive 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. 51, N, ak), the inner, the entoderm (N, ik). Ani- 

 mals 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 proliferation 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. 

 51, N). Animals with three germ-layers are said to be triplo- 

 blastic. 



The tissues developing from the germ-layers are, in part, as 

 follows. From the ectoderm arise the epidermis, epithelium of 

 various organs, and the nervous system; from the mesoderm 

 come the muscles, connective and supporting tissues, and blood 

 and blood-vessels; the entoderm becomes the epithelium of the 

 digestive tract, pharynx, and respiratory tract. 



Ccelom. — The ccelom is a cavity in the mesoderm lined by 

 an epithelium; into it the excretory organs open, and from its 

 walls the reproductive cells originate. There is no ccelom in 

 the lower Metazoa, but one is present in all the more complex 

 animals. As shown in Figure 51, N, 0, it arises in a typical 

 animal as cavities of the mesodermal pouches which form from 

 the primitive alimentary canal ( N, dh) . The outer mesodermal 

 lining of the ccelomic cavities is called the somatic epithelium (O, 

 mki), and the inner the splanchnic epithelium (0, mk 2 ). The 

 importance of the ccelom both morphologically and physiologi- 

 cally will be discussed later. 



