THE OASTEULA. 



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 ,,,ILI/JULIII, 



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- Fu . 40 _ U; Cell colony of young ,-,,., x 

 ther increase of surface brought <;i<>i'tito,- (after stein), i, Biastosphcre 



sluice of an Acalepha larva (Aureliu 



about by secondary imaginations tili . ifa) _ c , Gastnila stage of 6 ; EC, 

 and partly from the appearance of Ectoderm; En, En.ioderm; o, Blasto 

 some intermediate tissue placed be- P ve < mouth of <*astruia). 

 tween the two primary layers. The secondary invaginations perform 

 special functions and give rise to glands; while the intermediate 



