No. 3-] DEVELOPME.XT OF MARIXE SPOXGES. 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, PI. 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 PI. XVII, Fig. 45, it would 

 seem that the two canals, can.' and can!', have but lately met ; 

 and in Fig. 44 the canal, can.", has made connection with the 

 subdermal cavity, s. d. c. 



A more comprehensive idea of the formation of subdermal 

 cavities and canals may be obtained from a study of surface 

 views. In PL XVIII, P^ig. 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 PI. XVIII, Fig. 54, all lie 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 Monbranc. — 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, d. nicin., 

 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. PI. 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, 



