No. 3.] DEVELOPMENT OF MARINE SPONGES. 303 
56,57). fter the sponge takes on the round cake-like shape, 
the mes-entoderm becomes divided into two regions (Fig. 55), 
a main central region in which the formative cells are more or 
less rounded and pretty densely packed, and a peripheral zone 
in which the formative cells become branched and amoeboid, 
and in which they are loosely packed. The distinction between 
the two regions is conspicuous in Fig. 55, drawn under a low 
power; and the manner in which the peripheral zone is formed 
is clearly seen in Fig. 56 (surface view of a part of the 
peripheral region of a young sponge). Further development 
bestows on the peripheral zone the character of an exquisite 
intercellular network (see f. ¢., Fig. 57, surface view of a small 
part of the peripheral region of a young sponge). In the 
sponge represented in Fig. 57, the cells of the network are of 
about the same size as the formative cells in the rest of the 
body ; but as the sponge grows older, the cells of the peripheral 
zone grow smaller, many of them becoming delicate spindle- 
shaped cells. The peripheral zone after it has assumed this 
character, is shown more or less well in all the sections of older 
sponges figured (see Pl. XVII, Fig. 44, and especially Pl. XVIII, 
Fig. 49, the peripheral part of a section such as Fig. 44). The 
processes of many of the cells run directly into the ectodermal 
membrane, and strongly suggest an intercellular connection 
between the ectoderm and mesoderm in this region. 
When the ectodermal membrane grows out round the periph- 
ery of the sponge, the peripheral zone of mes-entodermic 
cells, which is already differentiated from the central mass, 
begins to push out lobes and processes within the membrane 
which is at its inner edge obviously composed of two layers 
(Fig. 49). This brings it about that the inner mass of cells 
loses the smooth contour of younger stages (Fig. 55), its 
edge becoming, instead, jagged and irregular (Fig. 58). The 
changes of shape which the sponge undergoes at this time are 
due to the fluctuations of the edge of the mes-entoderm mass, 
not to amoeboid movements of the ectoderm cells. Sponges 
are sometimes found in which the mes-entoderm has pushed 
out processes of very considerable length between the layers of 
the membrane. Such a sponge is shown in Pl. XVIII, Fig. 54. 
