256 EDWARD A. MINCHIN. 
To resume, then, this membrane may be described as a 
delicate network composed of two layers of cells with a 
minute quantity of jelly between them. The inner layer 
becomes directly continuous with the layer of collared epithe- 
lium composing the endoderm of the sponge. I have not ob- 
served with certainty any form of cell intermediate between the 
flattened cell of the sieve membrane and the columnar collared 
endoderm cell. The round cells often seen at the junction of 
the two, as in fig. 13, appear to me to be ordinary collared 
cells cut obliquely. Similar appearances can be seen in any 
spot where the section is not accurately radial to the wall of 
the tube. The outer layer of the sieve membrane becomes 
similarly continuous with the ectoderm. To discuss the 
morphology of this sieve membrane it is necessary to know the 
homologies of the layer composing it. Three alternatives are 
possible; either the inner layer is endoderm and the outer 
ectoderm ; or both layers are ectoderm; or both are endoderm. 
I think the third hypothesis may be dismissed at once, and 
that it lies between the first two. The question could only be 
solved satisfactorily by a study of the development of the mem- 
brane, which I have not been able to make, but I believe 
certain facts point very strongly to the first hypothesis being 
true, i.e. to the inner layer of the membrane being composed 
of endoderm, the outer of ectoderm. In the first place, in a 
growing colony of this sponge, there are two ways in which a 
new osculum may be formed. The first way is by actual divi- 
sion of an osculum into two. Fig. 2 represents two oscula, 
recently formed, I have no doubt in this way. It is a process 
similar to that described and figured by Schulze in Farrea 
occa,! a sponge which grows in a manner very similar to this 
Ascetta. The second way in which an osculum could arise 
would be by a cecal diverticulum growing out from the side of 
one of the sponge tubes, which after growing to a certain 
length, becomes perforated distally to form an osculum, very 
much in the manner in which new individuals are budded in a 
1 «Monograph of the Hexactinellida,” ‘“ Challenger” Rep. Zool.,’ vol. 
xxi, pl. Ixxii, figs. 1—3. 
