THE ORGANS. 



35 



onGj or endoderm, whicli surrounds a primitive enteric cavity. 

 The two layers pass into one another at the oral opening' which 

 leads into the cavity. The two cell-layers, which form the body 

 of such an organism, furnish the conditions under which it is 

 possible for it to lead an independent animal existence. The outer 

 one is the organ of support, and may be converted into an organ 

 of locomotion if it gives rise to cilia, and may be the seat of 

 respiratory functions also. In so far as it per- 

 ceives the state of the surrounding medium it 

 is an organ of sensation too. The inner layer 

 is nutritive in function, produces changes in 

 the food which is taken in, and allows what 

 can be assimilated to pass into its cells ; and 

 these in their turn feed the outer layer of 

 cells. What is useless is passed out again by 

 the same opening as that by which it entered. 

 As the functions of the two layers are dif- 

 ferent the special characters of the morpho- 

 logical elements which compose them are 

 different also ; we need only call attention 

 now to the much greater size in most cases 

 of the cells of the endoderm, as compared with 

 the cells of the ectoderm. 



This grade of organisation is to be seen 

 in some of the lower divisions of the Animal 

 Kingdom (Coelenterata and Vermes), where it 

 represents a lowly stage of development. 

 Indications of it are to be seen even in the 



higher divisions. This form has been called the Gastrula, on 

 account of the dominant development of the enteron. Starting 

 from the hypothesis that forms agreeing with a G-astrula in all 

 essential points were the precursors of all the higher forms of 

 animal organisation, a Grastreea-form resembhng the Gastrula has 

 been regarded as the primitive ancestral form of all animals. This 

 Gastraea theory is based, first, on the existence of independent 

 animal forms which resemble the Gastraea; secondly, on the fact 

 that the embryonic body which commences with a Gastrula, does 

 not, in the lower divisions, rise very much above it, so that 

 even apparently considerable complications of the organism can 

 be traced back to the existence of these two layers of the body ; 

 thirdly, the presence of these two layers of cells, forming the 

 ectoderm and endoderm, as a general, constant, and therefore regular 

 pheenomenon, even in the higher divisions of the Animal Kingdom, 

 as well as their constant relation to the same functions, is a fact of 

 the greatest importance for the hypothesis in question ; indeed the 

 occurrence of these layers as the so-called germinal layers, 

 which make up the embryonic body, cannot be rightly understood 

 without a reference to a hypothetical Gastraaa-form. This hypo- 

 thesis may therefore be regarded as justified. 



D 2 



Fig. 15. Diagram to 

 represent the first dif- 

 ferentiation of the or. 

 ganiam into ectoderm 

 and endoderm, and the 

 formation of a digestive 

 cavity, a Mouth. 6 En- 

 teric cavity, c Endo- 

 derm, d Ectoderm. (In 

 transverse section.) 



