410 M. E. Pcrn'er 0?? the 



Hilaire, who termed them analogies ; it is based on the pi-in- 

 ciple of connexions — that is to say, that it rests on tlie relative 

 position of the organs in the adult state and on the identity 

 of their mode of succession during the embryonic period, to 

 the exclusion of all consideration of function or of structure. 

 The endoderm being essentially that which is within, the 

 ectoderm that which is without, as indicated by their names, 

 it is clear that there is neither endoderm nor ectoderm in a 

 hollow body, such as the most simple form of sponge-larva, 

 formed of a single layer of cells, and that, in order to 

 remain in conformity with the precise language of compara- 

 tive anatomy and embryogeny, it will be necessary, if one 

 half of this body is invaginated within the other, to apply 

 the designation entoderm to that which becomes internal and 

 to term exoderin that which remains external. The propo- 

 sition of M. Delage might therefore be enunciated simply as 

 follows: — 



The larva of Sponges is a7i ellipsoid., one cap of which is 

 formed by fagelliferous, the other hy granular cells ; the cap of 

 flagelliferous cells is invaginated into the other and constitutes 

 the entoderm. 



In designating this cap by the name exoderm, in homolo- 

 gizing it with the exoderm of the rest of animals, we run 

 counter to the very definition of homologies, for it is implicitly 

 agreed that the character of the exoderm is derived not from 

 its position, but from the form of the anatomical elements 

 composing it. 



Ihe character invoked by M. Delage amounts therefore to 

 saying that the entoderm of Sponges is formed of flagellife- 

 rous and their exoderm of granular elements ; it is a histo- 

 logical character^ like that which is derived from the presence 

 of the choanocytes in the ciliated chambers — nay, more, it is 

 exactly the same character., since M. Delage recognizes that 

 the ciliated chambers originate from his supposed exoderm, 

 and the question of the position of the Sponges remains pre- 

 cisely at the point at which it had been left by his prede- 

 cessors. The apparent progress results simply from the 

 ci'edence once more accorded to the metaphysical theory of 

 the embryonic layers and of their j)redestination — a theory in 

 some sort retroactive., like all those which claim to apply to 

 the lower animals conceptions derived from the study of the 

 higher animals, often even of single vertebrates, and based 

 upon structural features which are the result of the activity of 

 primitive animal forms, or are considered as such, but are not 

 yet realized in them. E\ery animal, it is said, is at first a 

 gastrula, composed of an exoderm and an entoderm : the 

 generalization is gratuitous; the exoderm and entoderm are 



