PROF. DELAGE ON SPONGES. 59 



the invai^inated layer, and laying no stress on the fact that the 

 invaginated layer shows the characters of an ectoderm and 

 ince versa. 



There was indeed a strong reason to do so. This reason was 

 that in the siliceous Sponges, which are far more numerous than the 

 calcareous, the flagellated layer was believed to form the epidermis 

 of the grown-up animal, and the granulated cells its digestive 

 epithelium ; and that it seemed impossible to assume that a dif- 

 ference could exist between such kindred animals as calcareous 

 and siliceous Sponges in so important a feature as the relation 

 between the organs of the adult and the layers of the embryo. 



Yet, doing so was by no means to get rid of the difficulties. 

 There still remained that the layer named ectoderm in the cal- 

 careous Sponges had the histological characters of the one named 

 endoderm in the siliceous, and vice versa, a difference which remained 

 unexplained. 



Your celebrated embryologist, Francis Balfour, had a prophetic 

 view of the matter when he wrote : " Considering the difficulties of 



observation, it appears better to assume that some observations 



are in error, rather than that there is a fundamental want of uni- 

 formity in development amongst the Spongida." 



I have been so fortunate as to show the truth of the idea and to 

 demonstrate that the course of the development is fundamentally 

 the same in the siliceous as in the calcareous Sponges. In the 

 former as well as in the latter, the flagellated cells get inside to 

 become the choanocytes and the granulated cells pass outside to 

 form the epidermis, a discovery which has been verified many times 

 since by other zoologists, Weltner, Noldeke, and above all by 

 O. Maas, whose extensive and accurate researches have very much 

 extended and improved our knowledge on the subject. Recently 

 Mr MiNCHIN in Leiicosolenia and O. Maas in Oscarella have 

 succeeded in finding that these Sponges, which remained yet as 

 exceptions to the general rule, in reality develop according to the 

 same law as the rest of the Spongida. 



Now the matter is settled in such a way that the following 

 conclusion cannot be gainsaid : tJie layer which in larval Sponges 

 has the histological characters of an ectoderm has the evolution of an 

 endoderm by becoming invaginated and forming the digestive epi- 

 thelium ; and the layer zvJiicJi has the histological characters of an 

 endoderm evolves as an ectoderm by ronaining or passing outside 

 and forming tJie epidermis of the adult. 



I lay stress on this that, expressed in those words, the fact is 

 above all discussion. And I say again : is not such a difference 

 important enough to check all attempt to place the Spongida in 

 the same ph\'lum as any other Metazoa, and especially as the 

 Coelenterata ? 



So, we could stop here. 



But, there remains a last question. We have just seen that 



