36 COMPARATIVE ANATOMY. 



We recognise then the Gastrasa as a fundamental form, and 

 discover in the differentiation of two layers corresponding respec- 

 tively to endoderm and ectoderm, which are present even in the 

 highest grades of the Animal Kingdom, facts which point to such 

 a Gastraea stage, and are due to it. But it must not be at all sup- 

 posed that we have advanced farther than just on to the threshold 

 of a knowledge of these relationships. The definitive explana- 

 tion of many of the points which have considerable importance in 

 this matter is still far distant, and but little light has yet fallen on 

 even such apparently simple points as the origin of the Gastrula 

 and its two layers. It is a question whether the form which pre- 

 cedes the Gastrula is a one-layered vesicle — that is, whether the 

 two layers of the body are due to the endoderm being formed by 

 invagination ; or whether the endoderm is developed from a pi'imitive 

 internal cell-layer delamination. And again, whether the two con- 

 ditions that have been observed are independent of or derivable from 

 one another. Further investigations will have to settle all this, and 

 our judgment must therefore be proportionately reserved until such 

 investigations have been made. 



§ 29. 



The two layers of which the body of the lower animals is made 

 up during their early stages, and which are, in the higher divisions, 

 represented by the germ -layers — that is, the ectoderm and 

 endoderm — give rise to an intermediate layer or mesoderm, in the 

 formation of which the other two apparently take an equal share. 

 But it is not yet definitely known what share each takes, since the 

 earliest processes of the differentiation of the rudiments of the body 

 still require much careful investigation, and, moreover, they do not 

 present the same characters in all cases. These three layers appear 

 directly after the segmentation of the ovum in the higher animal 

 organisms, and are coincident with the first traces of histological dif- 

 ferentiation. They represent the outline of the organism in the condi- 

 tion of a germ, and from this the whole organism by differentiation 

 evolves itself. 



These rudiments of the body exhibit great modifications in the 

 higher divisions of the Animal Kiugdom, and the stage which is 

 represented by the Gastrula form is more difficult to make out in 

 proportion as the differentiations through which the organism has 

 to pass are more considerable; but the principal features can be 

 easily recognised as identical in all cases. The outer germinal layer 

 (deric layer or ectoderm) forms the outer limiting layer of the body, 

 and the inner (lower) germinal layer (enteric layer, glandular layer, 

 or endoderm) the foundation of the gut or enteron. The middle layer 

 (mesoderm) afterwards arises between them. 



As the ectoderm and endoderm are the first organs marked off 

 in the course of development, the germinal layers are to be regarded as 

 primitive organs, which have been transmitted from the earliest 



