No. 3.] DEVELOPMENT OF MARINE SPONGES. 347 
Hircinia there is the same indication as in Tedanione, that the 
mesoderm cells bring food to the growing egg. Until segmenta- 
tion begins the egg is thickly surrounded by mesoderm cells, Pl. 
XXIV, Figs. 116 and 119, which have large bodies full of fine 
yolk granules. Amongst these cells are scattered a few with 
very coarsely granular bodies, Fig. 119. The mesoderm cells 
are applied so closely to the follicle that the latter cannot be 
distinguished as a definite layer. All that can be said is that 
at this time the egg is surrounded by closely packed cells, 
arranged irregularly in two or three strata, the inner stratum 
doubtless representing the follicle of the very young egg, 
while the outer strata consist of mesoderm cells which have 
applied themselves to the follicle. The inner stratum of cells 
is, as may be seen in Fig. 116, often very irregular, and there 
are certain indications that the follicular cells are sometimes 
pinched off and engulfed by the egg — notice the very protu- 
berant rounded cells projecting into the substance of the egg. 
Whatever be the precise manner in which food is conveyed 
from the surrounding cells to the egg, it seems pretty certain 
that food is so conveyed, and that this is the object of the 
migration of so many mesoderm cells to the immediate neigh- 
borhood of the growing egg. The egg at the time when it 
reaches full size is still surrounded by several strata of cells, 
Fig. 119. The inner stratum, however, soon becomes trans- 
formed into a follicular membrane, consisting of flattened but 
still rounded and protuberant elements, Pl. XXIV, Fig. 120. 
The cells of the outer strata gradually wander away, leaving 
the mesoderm round the segmenting egg not more abundantly 
supplied with cellular elements than is the mesoderm in most 
parts of the sponge. 
The ripe egg of Hircinia is closely packed with yolk spheres 
of a large size, Fig. 119, which make their appearance in the 
developing egg-cell after a fashion essentially similar to that 
already described for Tedanione. In the finely granular body 
of the very young egg-cell, yolk spheres appear which are at 
first small, Figs. 115, 115!, but which gradually increase in 
size, becoming at the same time more closely packed. In the 
egg shown in Fig. 116, the bulk of the yolk is still composed 
