TILE GASTBULA. 



49 



it became necessary that they should be divided, so as to bound two 

 surfaces, into an external and an internal layer ; the one forming 

 the outer wall of the body and known as ectoderm, and the other 

 lining the central cavity (digestive _ 



cavity) known as endoderm; these 

 two layers being continuous with 

 one another at the opening of the 

 central digestive cavity, or mouth 

 opening (fig. 40 c). The cells of 

 the two layers, in correspondence 

 with the difference in their function, 

 possess a different structure. Those 

 of the outer layer, which carry on 

 the animal functions, are usually 

 cylindrical ciliated cells containing 

 a pale albuminous substance ; those 

 of the inner layer are more rounded 

 and of a darkly granular aspect; 

 they may also bear cilia for the 

 movement of the contents of the 

 cavity which they line. In actual 

 fact we find this form, which from 

 a physiological standpoint is the 

 simplest organism with cellular dif- 

 ferentiation that we can conceive 

 of, realised in the two-layered " gas- 

 trula," which appears in the de- 

 velopment of almost all groups of 

 the animal kingdom as a free- 

 swimming larva, and to which the 

 adult sexually mature Crelenterate 

 closely approximates. 



As the organism increases in 

 size, additional complications ensue. 

 These result partly from a still fur- 

 ther increase of surface brought 

 about by secondary invaginations 

 and partly from the appearance of 

 some intermediate tissue placed be- 

 tween the two primary layers. The secondary invaginations perform 

 special functions and give rise to glands; while the intermediate 



FIG. 40. a, Cell colony of young Vohox 

 Glolator (after Stein). I, Blastosphere 

 stage of an Acalepha larva (Aurelia 

 Aurita). c, Gastrula stage of b; Ec t 

 Ectoderm; En, Endoderm; o, Blasto 

 pore (moutli of Gastrula). 



