No: 3.] DEVELOPMENT OF MARINE 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 ante, p. 317, 359. 
Formation of epidermis. —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 azte, 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, Iam prepared to believe in, some of my own observa- 
tions suggesting, though by no means proving, the occurrence 
of such a phenomenon (axz/e, 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 (anzée, 
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 
