58 SECOND GENERAL MEETING. 



sucJi differences that — though lue reviain unable to point out the 

 precise value of the characteristics of phylum s — ive could nevertheless 

 be sure that these differences are more important than are required to 

 characterise different phylumsf 



I think we may answer Yes ! Such differences are to be found 

 in the development. 



If you examine the common larval form of a calcareous 

 Sponge, the one known by the name Amphiblastula, you will find 

 nothing puzzling in it. There is an upper part composed of cells 

 long, slender, compressed, bearing each of them a long flagellum, 

 and containing a pale cytoplasm devoid of food granulations. 

 The under part, on the contrary, is formed of cells big, roundish, 

 devoid of flagellum and abundantly furnished with granular food 

 particles which make their cytoplasm dark. When you follow this 

 larva in its development from the Qgg, you see that this &gg, after 

 having been segmented in two meridional planes into four equal 

 blastomeres, is, at the third stage, divided in a transverse plane into 

 two sets of cells, one upper of four small protoplasmic cells and one 

 inferior of four large cells, rich in food yolk. The former set gives 

 rise, by an active process of division, to the flagellated part of the 

 larva, while the cells of the inferior group divide more sluggishly to 

 give the granulated part of the larva. 



This being so, I suppose that you are (what is really the case) 

 very learned zoologists, perfectly acquainted with the general 

 subject in embryology, but (what is not the case) knowing nothing 

 of the development of Sponges, and I say : " Please, tell me where 

 is the ectoderm, where is the endoderm, and try to foresee what part 

 of this blastula will be invaginated to form the digestive epithelium, 

 what part will remain outwards to form the skin?" Surely your 

 answer will be : " The flagellated layer is the ectoderm, the large, 

 granulated cells are the endoderm and will invaginate themselves 

 in the former." 



And yet it is just the contrary ! 



Were it the larva of any other animal, you were right ; it being 

 the larva of a Sponge, you are wrong. The flagellated cells get 

 inside to form the digestive epithelium, and the granulated cells 

 remain outwards to form the epidermis. 



In the Coelenterata, the invagination takes place in the same 

 normal way as in the other Metazoa. In the Spongida, the inva- 

 gination is reversed. 



Is it not that such a difference as this is more important than 

 that required to characterize a phylum ? 



It seems that there could be no discrepancy of opinion in such 

 a case as that. 



However it is not so. Zoologists, generally, do not admit this 

 inversion of the normal invagination which seems so evident. They 

 prefer to take another course and to admit that the invagination 

 takes place in the normal way, giving the name of endoderm to 



