SPONGI^J. 
8 Spang, 
the flat, cake-like larva. The mesoderm now arises by the inwandering 
of cells derived by fission from the endoderm. This appears to follow 
from the fact that the earliest observed mesodermal cells occur near the 
endoderm, that the young mesoderm cells are structurally more similar 
to the endodermal than to the ectodermal cells, that forms were observed 
which seemed to suggest an alteration of endodermal into mesodermal 
cells, while ectodermal cells never presented these appearances. 
The ectoderm forms regular little feet for the attachment of the 
Sponge ; these commence as short conical processes, which extend into 
irregular tentacles, and run out in various directions seeking points of 
attachment ; some broadon out into round, sucker-liko feet, the ecto- 
dermal cells on the face of which are longer than usual, and resemble 
gland cells ; probably they secrete a cement for attachment — indeed, such 
a cement appears to cover the surface to which they adhere. A difference 
between the cells forming the margin of the blastopore could not be 
proved by thin slices, but they appear to be larger than the other ectoderm 
cells. 
(h) Definite formation of flagellated chambers. — The flat, cake-like 
larva* increases chiefly in height, and becomes hemispherical, owing to the 
filling of the gastral cavity with fluid (sea-water, containing some dis- 
solved albumen). The ectoderm becomes smoother, irregular processes 
of attachment disappear, the cells become flatter, and pass into pave- 
ment epithelium, except those of the basal part, which, in some other 
places besides the feet, retain a cubical form. Inside the inner circle of 
diverticula, at the summit of the gastrula, the endoderm remains un- 
folded (the oscular plate); at the base of the larva it is also unfolded 
(basal endodermal plate). The flagellated chambers have now acquired 
complete histological differentiation and a definite spherical form ; their 
communication with the gastral cavity is constricted to a wide circular 
or oval opening. Pores now form as short invaginations of the ectoderm, 
though in many cases the distal end of the flagellated chamber and the 
surface of the ectoderm approach so closely that no invagination is 
needed, and simple perforation occurs : in other cases— and here, as in 
Placina t these are the rule — a single pore canal serves two chambers 
and opens on the surface in a single aperture between them ; such are 
formed by invagination. Isolated flagellated chambers have been observed 
on the basal layer of endoderm. 
( j ) Formation of the oscule. — The ectoderm at the summit of the 
hemispherical gastrula rises up from the underlying endoderm, and a 
continually increasing quantity of jelly gathers between them: thus 
arises a solid process, which rises vertically upwards, but otherwise 
resembles one of the marginal tentacular processes for attachment. Soon 
a diverticulum from the endoderm grows into this, the intervening jelly 
is displaced, the ecto- and endoderm meet together at the summit, fusion 
aud perforation follow, and the oscule is the result. 
(Jo) Theoretical part. — The development of Oscarella is typical ; the 
amphiblastula of the Galcarea is coenogenetic. The iuvagihated layer 
of the gastrula is the nutritive layer. The ancestral form of Sponge was 
a flat gastrula, which lived, crawling mouth downwards, over the surface 
