306 WILSON. [Vou. IX. 
cells too take part in this intercellular network. In Pl. XVII, 
Figs. 39 and 42, are shown parts of vertical sections through 
two sponges in the same stage as Fig. 38. Owing to the 
quantity of water in the sponge at this age, the tissues are 
extremely delicate and gelatinous, and the intercellular net- 
work in the best preparations is naturally more or less broken, 
with many of the cells fallen out of their proper places. That - 
the canals and subdermal cavities arise as great intercellular 
spaces or lacunae in the mes-entoderm, can easily be seen in 
these figures. The lacunae when first established have no 
definite walls, but are merely surrounded by ordinary undiffer- 
entiated mes-entoderm cells, caz., Fig. 39. The cells imme- 
diately surrounding the cavity then begin to flatten, throwing out 
lateral processes in such a way as to form a more or less com- 
plete wall, in which, however, the component cells are of very 
irregular and diverse shapes, Fig. 42, and caz., Pl. XVIII, Fig. 
47 (small part of a section, such as Pl. XVII, Fig. 44). The 
lining cells continue to flatten, ultimately forming a continuous 
investment of epithelioid cells, so thin indeed that they consti- 
tute nothing more than a nucleated membrane, ca. w., Fig. 47. 
In Fig. 44.there is shown a small canal, can.', in which a part 
of the wall has reached the condition of a nucleated membrane, 
while the other part is still composed of cells which have not 
yet flattened out to any great degree. Cavities are developed 
everywhere directly beneath the upper surface, and there con- 
stitute, as has been said, the system of subdermal cavities, Pl. 
XVII, Figs..38, 44, and so; Pl. X VILL Figs. 48,.51, 52,.and.52: 
The spaces formed deeper in the tissue of the sponge become 
the canals. The number of subdermal cavities and canals is at 
first relatively small, so that the space occupied by the meso- 
derm is comparatively great, Pl. XVII, Figs. 38 and 44. But 
as new canals are formed, and as the cavities and canals 
gradually connect with one another, the mesoderm becomes 
reduced in quantity, and before long assumes the adult con- 
dition, in which it consists of uniformly thin trabeculae 
separating the various canals. The increase in the extent of the 
series of cavities may be seen in a comparison of Pl. XVII, Fig. 
44, with sections through older sponges, Pls, XVII and XVIII, 
