450 SCIENCE PROGRESS. 



where the development required re-examination in the hght 

 of these views, in order to determine whether a similar re- 

 version of the layers, not hitherto suspected, did occur» 

 or whether the recorded statements were correct. These 

 cases were, besides Oscai-ella with its somewhat peculiar 

 development, P/akina, Halisarca, and the Ascons. 



Since my review of these questions in this journal, three 

 works have been published upon sponge development, those 

 of H. V. Wilson (1894), Noldeke (1894), and myself ( 1896). 

 To take the last first, I have shown that in the Ascons the 

 layers become reversed in a manner similar to that de- 

 scribed by Maas and Delage, and that it thus becomes 

 possible to homologise the parenchymula and amphiblastula 

 types of development, the internal non-ciliated cell mass of 

 the former being homologous with the posterior non-ciliated 

 cells of the latter, and giving rise in the same way to the 

 dermal layer of the adult. 



The observations of Wilson on the other hand, dealing 

 with several species of Monaxonida are interpreted by him in 

 an opposite sense to the views expressed by Delage and Maas 

 after a study of the same class of objects. Wilson believes 

 that the ciliated layer of the larva becomes the "ectoderm," 

 that is to say the dermal layer of the adult, and derives the 

 collar cells, the "endoderm," from the inner cell mass of 

 the larva. He thus brings us back once more to the old 

 views of sponge embryology, which sought in vain to con- 

 struct a uniform type of sponge development from a chaos 

 of contradictory and irreconcileable observations. 



Not only must I confess myself biassed against Wilson's 

 views on general grounds, that is to say, on account of the 

 impossibility of homologising in any other way the am- 

 phiblastula and, I may add, parenchymula larva with other 

 types ; not only am I greatly inclined to regard Wilson's 

 statements as less convincing than those of Maas and 

 Delage ; but I consider that Wilson's own recorded obser- 

 vations are not at all in harmony with his interpretations of 

 them. Wilson has objected strongly to his figures being 

 taken as evidence against his views, a somewhat curious posi- 

 tion to take up if his figures profess to be accurate ; we can 



