302 WILSON. [ VoL. TX. 
here and there, but in which I could not make out the cell 
boundaries. The ectoderm covering the surface of attachment 
is noticeable for the deeply-stained thickenings found in it in 
comparative abundance (g7 7h. in Fig. 38, and in Fig. 39, 
part of vertical section through recently attached sponge). 
These thickenings are shaped and distributed as if they might 
be nuclei, surrounded by an accumulation of protoplasm, but 
the stain reveals nothing but a homogeneous mass. These 
bodies are found, often of large size (Fig. 44), in the ectodermal 
membrane surrounding the sponge, of which I shall speak 
presently. In both situations they give the impression of 
degeneration products. 
The little sponge, when the flattening out is completed, has 
a smooth and nearly circular outline, the mes-entoderm extend- 
ing quite to the edge of the body. Pl. XVIII, Fig. 55, showsa 
surface view of such a stage. The ectoderm at the edge of the 
sponge soon begins, however, to grow out in the shape of a 
thin membrane which completely surrounds the sponge, extend- 
ing outwards toa considerable distance. This membrane, ec. m., 
is shown in Pl. XVII, Figs. 38, 39, 44 (vertical sections), and 
in Pl. XVIII, Figs. 56, 58 (surface views). Nuclei can be made 
out here and there in it, but the cell outlines are indistinguish- 
able. Before continuing the description of this membrane, it 
will be necessary to say a word or two in regard to the mes- 
entoderm of the recently attached sponge. 
The first change which the mes-entoderm of the larva under- 
goes during metamorphosis may be gathered from a comparison 
of Pl. XVI, Figs. 20, 30, and Pl. XVII, Figs.36, 37. — It will be 
seen that the formative cells increase greatly in numbers, and 
become distributed uniformly through the body. The pale, 
densely packed polygonal cells which occupy the posterior end 
of the swimming larva, gradually disappear, probably becoming 
transformed into the more independent and consequently 
rounded formative cells (see periphery of Fig. 36). The slender 
spindle cells which occupy the anterior end of the swimming 
larva, are in their turn distributed through all parts of the body. 
The next change in the development of the mes-entoderm 
can best be studied in surface views (see Pl. XVIII, Figs. 55, 
