THE EMBRYOLOGY OF THE PORIFERA. 211 



the larva it most nearly resembled, namely, the partially 

 ciliated larva of Silicispongise ; for the cell layers which 

 resembled each other in appearance and position in the two 

 types of larvae were apparently precisely opposite in their 

 homology. 



With matters in this condition, an entirely new light 

 has been thrown upon the question by the independent 

 investigations of Maas in Germany, and Yves Delage in 

 France. These two authors, though differing greatly in 

 details, are agreed upon certain fundamentally important 

 points, namely, that in the solid, planula-like larva of 

 siliceous and horny sponges, the flagellated external cell 

 layer, contrary to former statements, gives rise to the 

 collar cells of the chambers of the adult sponge, while 

 the inner mass of cells gives rise to the flattened epithel- 

 ium lining the outer surface and the canal system, as well 

 as to the whole of the so-called mesoderm of the sponge. 

 Or, to put it differently, if we agree to term the collar- 

 cell layer of an adult Ascon, or any other sponge, the 

 endoderm, its flattened epithelium ectoderm, and the inter- 

 mediate layer mesoderm ; then, in the larva of siliceous 

 sponges, the flagellated cell layer is, as in the amphi- 

 blastula of Calcarea, the endoderm, while the inner mass 

 of granular cells represents an ecto-mesoderm. We thus 

 find a fundamental agreement, instead of difference, between 

 the type of larva, most common in calcareous, and siliceous 

 sponges respectively. On the other hand, the apparently 

 paradoxical fact of endoderm completely or partially sur- 

 rounding ectoderm, separates them completely from other 

 Metazoan, and especially from Ccelenterate larvae, and for 

 this reason it will be best to avoid at the outset such terms 

 as ectoderm and endoderm, as more or less implying homo- 

 logies outside the group, and to consider simply the plain 

 facts of development, as stated by Maas and Yves Delage. 



The investigations of Maas commence with the seg- 

 mentation of the egg and extend over the larval life and 

 metamorphosis up to the young sponge, while Delage has 

 studied the post-larval development only. The species 

 studied by Maas are Esperia {Esperelld) Lorenzi O. S. and 



