STUDIES ON THE COMPAEATIVE ANATOMY OE SPONGES. 353 
cells lying near the outside of the embryo have two or three 
nuclei (fig. 22), and very rarely also even the ectodermal cells 
appear to have two nuclei (fig. 20). At a later stage (fig. 23) 
the entire amoeboid cell is seen to have become indistinct in 
outline, and in place of one large cell we have an aggregation 
of very minute spherical bodies about 00025 mm. in diameter, 
each with a dark spot in its centre ; but each aggregation still 
retains the form of the original amoeboid cell. In the same 
sections which exhibit this condition many of the amoeboid 
cells appear to have become rounded, their contents having 
arranged themselves around a central cavity (fig. 23,/. c.), so 
that we have a hollow chamber lined by small, spherical cells. 
These chambers I believe to be the young flagellated chambers. 
They are certainly very different in structure from the flagel- 
lated chambers of the adult Sponge, and only about half the 
size, measuring about 0024 mm. in diameter, but the dif- 
ferences are easily accounted for by their embryonic condition. 
I have not been able to trace the development of the chambers 
any further, nor is it to be expected that the collars and 
flagella would be developed before the young Sponge was set 
free and required them. 
Coincidently with the formation of the flagellated chambers 
in the manner thus described, a slit-like invagination appears 
on the surface of the young Sponge, and it is chiefly, if not 
solely, around this invagination that chamber formation takes 
place. This invagination is probably the commencement of a 
communication between the chambers and the exterior. Un- 
fortunately, I have only obtained a single embryo which is 
sufficiently advanced to show the formation of the flagellated 
chambers and the slit-like invagination from the exterior, but 
I see no good reason for doubting the normality of the pheno- 
mena above described. 
The mode in which the embryos of Stelospongus escape 
from the parent is still an enigma. It may be that by further 
increase in size they rupture the walls of the oscular tubes in 
whose immediate proximity they lie, and are then forcibly 
ejected with the outgoing stream of water; or it may be that 
