No. 3-] DEVELOPMENT OF MARIXE SPONGES. 375 



chambers. Simultaneously the epidermic elements come to the 

 surface and fuse with one another to form a complete mem- 

 brane, the definitive epidermis. The amoeboid cells become 

 the wandering cells of the adult mesoderm, while a part of the 

 intermediary cells form the epithelium of the canals, the rest 

 becoming the stationary elements of the mesoderm. These 

 conclusions differ, it will be seen, in some important respects 

 from those presented by the author in his preliminary notes 

 (Comptes Rendus 1890, 1891), cited ^«/^, p. 317, 359. 



Foniiatioii of cpidcruiis. — In believing that the cells which 

 cover the posterior pole form a different part of the adult body 

 from the rest of the covering cells of the larva, I think Delage 

 is wrong. That no such distinction exists between these two 

 sets of the superficial cells of the larva, is made probable at 

 the very beginning where it is seen that the young embryo is 

 covered with a continuous layer of similar cells (columnar in 

 Tedania, 35, p. 576, and ante, p. 330), which subsequently 

 differentiate into the ciliated cells and the flattened ectoderm 

 of the posterior pole. Delage like myself is unable to offer a 

 satisfactory explanation of the peculiar character of this pole. 

 He does put forth the suggestion that it is due to a rupture in 

 the covering of ciliated cells, produced at a point of weakness 

 by the growth of the inner mass. But the observation I have 

 just cited upsets such an explanation. 



The immigration into the interior of a part of the ciliated 

 cells, I am prepared to believe in, some of my own obsen'a- 

 tions suggesting, though by no means proving, the occurrence 

 of such a phenomenon (iiiitc, p. 299). On the other hand I 

 am sceptical as to the existence of Delage's layer of epi- 

 dermic cells, not having found any such layer in the larvae 

 I have studied. I regret that my observations on the actual 

 transformation of the ciliated cells of the larva into the flat- 

 tened epidermis of the adult are so meagre, but such as they 

 are they are in harmony with the views of those writers (ante, 

 p. 301) who claim to have seen such a transformation, and not 

 with the views of Delage. It may be mentioned that Delage 

 finds the formation of the definitive epidermis to begin at the 

 anterior pole and gradually progress towards the posterior pole 



