EPITHELIOID MEMBRANES IN MONAXONID SPONGES 547 
themembranes had completely formed and the canals were entirely 
closed in. In fig. 5 one of the newly formed membranes, cm., 
with surrounding collenchyma and outlying choanosome is shown. 
When the cut is first made, the dermal membrane covering 
the rest of the sponge ends at the exposed surface with a sharp 
edge. On the cut surface itself are exposed in choanosomal 
regions, flagellated chambers, mesenchyme, and spicules; in the 
regions immediately round the larger canals, only collenchymatous 
mesenchyme. The mesenchyme everywhere includes branched 
cells freely interconnected, and free amoebocytes. The latter 
are scarce in the collenchyma. Collenchymatous mesenchyme 
is especially characterised, it will be remembered, by the large 
amount of watery intercellular substance and the considerable 
length of the cell processes. A recognisable new dermal mem- 
brane develops over the whole surface within a day. The edge 
of the old membrane remains distinguishable for some hours, 
but it applies itself closely to the more solid sponge tissue, sinking 
in to meet the latter where it had covered subdermal spaces, and 
after about 12 hours it is no longer recognisable. By this time 
it is in perfect continuity with the layers of mesenchyme cells 
stretching over the cut surface and which are developing into the 
new dermal membrane. 
We may now proceed to the detailed examination, by stages, of 
the developing dermal membrane and epidermis, using for study 
as explained above the membranes that develop across the open 
ends of canals and over collenchymatous regions. 
One hour after cutting. The cut surface is occupied by branched 
cells containing abundant and conspicuous granules. Even in 
a comparatively small area they exhibit slight differences of level. 
These cells are interconnected so as to form a fairly close network. 
Some very small spheroidal cells, probably metamorphosed collar 
cells, lie free here and there. Many of the superficial granular 
cells are thin and flattened. Below the surperficial cells lie several 
layers of essentially similar granular cells which are not flattened. 
They are interconnected with one another and with the super- 
ficial cells. The entire network formed by the granular mesen- 
chymal cells is closest at the cut surface and becomes more and 
more open as we go deeper below the surface. 
