No. 3.] DEVELOPMENT OF MARINE SPONGES. 307 
Figs. 48, 50, and 51. In the latter two figures the adult con- 
dition of the mesoderm has practically been reached (compare 
section of adult, Pl. XIV, Fig. 2). Communication between 
the various cavities is established by simple perforation of the 
intervening tissue, the cavities in question growing towards one 
another, and finally meeting. In Pl. XVII, Fig. 45, it would 
seem that the two canals, can.’ and can.", have but lately met ; 
and in Fig. 44 the canal, caz."", has made connection with the 
subdermal cavity, s. d. ¢. 
A more comprehensive idea of the formation of subdermal 
cavities and canals may be obtained from a study of surface 
views. In Pl. XVIII, Fig. 55, the earliest cavities are shown, 
as yet surrounded only by undifferentiated mes-entoderm cells. 
Two cavities in the same early stage of development are like- 
wise shown in Fig. 56. In the sponge drawn in Pl. XVIII, Fig. 
58, in about the same stage as Pl. XVII, Fig. 44, the cavities 
are numerous and a higher power would show they were lined by 
an epithelioid membrane. The cavities shown in this figure, as 
those in Pl. XVIII, Fig. 54, all he directly beneath the surface. 
Other deeper lying cavities are present, but these naturally are 
not obvious. The smooth rounded outlines of the cavities 
coupled with the extreme transparency of the overlying sponge 
tissue (dermal membrane) at first sight makes many of the 
cavities appear as oscula (Figs. 54 and 58), but examination 
soon reveals the membrane covering them. 
Dermal Membrane.— The portion of the sponge body which 
directly covers the subdermal cavities, develops into what is 
known as the dermal membrane. In its adult condition, @. mem., 
Pl. XIV, Fig. 2, and Pl. XVIII, Figs. 48, etc., it consists of three 
layers: on the outside the ectoderm, on the inside the epithelioid 
lining of the subdermal cavities, and between the two a layer of 
mesoderm consisting for the most part of slender spindle-shaped 
or fibre-like cells (comp. Pl. XIV, Fig. 4). The first stages 
in the formation of the membrane are shown in Pl. XVII, Figs. 
38, 39, 42. As the lining cells of the cavities flatten out, the 
superjacent mesoderm cells grow smaller and become trans- 
formed into spindle-shaped or branched cells, most of which lie 
in planes parallel to the surface. In the somewhat older stages, 
