258 EDWARD A. MINCHIN. 
Secondly, the collars of the endoderm-cells were compara- 
tively low, not more than one third the height of the cell, while 
in cells from other parts of the sponge in the same section the 
collars were more than half the height of the cell, (fig. 4). 
Thirdly, in the membrane itself, the cells composing it were 
less granular and opaque, appearing more protoplasmic, with 
very distinct nuclei. The first two of these points makes it 
probable that the osculum was not in full functional activity ; 
the third point shows that the cells were in a more primitive 
and less differentiated condition. Here then is just such an 
osculum as one would expect to find on the hypothesis that the 
sieve membrane arises as a breaking through of the gastric 
cavity to the exterior in several places and that the inner layer 
of cells composing it is endoderm, derived by flattening out 
of the collared endoderm-cells, while*the ‘outer layer is simi- 
larly ectoderm. Unfortunately I have observed no other inter- 
mediate stage. A curious point is the projection above the 
membrane of the wall of the sponge, forming the funnel-shaped 
expansion mentioned above. Here I may refer to Schulze’s 
well-known figures of the young S'ycondra raphanus.} 
These figures represent the young Sycon in an Ascon stage, and 
one might say that here we hada transitory Leucosolenia, with 
an osculum covered by a sieve membrane with only a single 
perforation. Round the edge of the osculum a fringe of 
spicules projects up. As one knows that projecting spicules 
in sponges are not really naked, one can easily imagine how 
from such a condition a rim like that in Ascetta could be 
formed. 
The conclusion is, then, that the sieve membrane we are here 
concerned with is formed by the gastral cavity breaking 
through to the exterior in several places'during development, 
and that its inner layer of cells is endoderm, the outer layer 
1 * Zeitschr. f. wiss. Zool.,’ xxxi (1878), Taf. xix, figs. 12, 13; or see 
Vosmaer, ‘ Porifera’ (Bronn’s Thierreich), Taf. {xxxi, ‘figs. 9,'10 ;> Balfour, 
‘Comp. Embr.’ 
From the condition figured by Schulze might easily arise either a sieve 
membrane or an oscular sphincter, or an ‘‘ Oscular-membran.” 
