302 WILSON. [Vol. IX. 



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 (pr. th. in Fig. '^?>, 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, shows a 

 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 to a considerable distance. This membrane, ec. w., 

 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 descripti6n 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. 29, 30, and PL XVII, Figs. 36, n. 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, 



