No. 3.] DEVELOPMENT OF MARINE SPONGES. 212 
in size they become arranged in a single layer, az. f. ¢.", Fig. 40. 
The surface of the flagellated chamber so formed gradually 
becomes smooth, and its shape, which may be of almost any 
character (an. f. c.", an. f. ¢."", Fig. 41, and az. f. c., Fig. 42), 
becomes spheroidal. 
The two methods of forming the flagellated chambers just 
described are distinct methods, though, as will be pointed out, 
one may be regarded as a modification of the other. That the 
two methods are distinct, that one is not a mere stage of the 
other, must be evident from the description. On the one hand 
we have solid masses of quite small cells, of a characteristic 
appearance, giving rise to a chamber; on the other, formative 
cells are found grouped in a hollow sphere (only in rare instances 
do these large cells form solid groups, for.c. g., Pl. XVIII, Fig. 
57), giving rise directly to a chamber. After the system of 
cavities has got well started, Pl. XVII, Fig. 44, and Pl. XVIII, 
additional chambers are formed, I think, exclusively after the 
second method; at least no hollow groups of formative cells are 
found, but, on the other hand, solid masses of small cells are 
comparatively abundant. 
In some few individuals the chambers are formed in yet 
another fashion. The cells of the just attached sponge may 
nearly all split up into fine cells, so that the mes-entoderm is 
transformed into a nearly uniform mass of fine cells, with a 
few larger (formative) cells scattered about here and there. In 
Pl. XVII, Fig. 43, is shown a part of a vertical section through 
such a sponge. In it one flagellated chamber, f. c., is marked 
out. In a sponge which happens to develop in this way it 
seems that the flagellated chambers must be produced simply 
by the appearance of cavities or lacunae in the tissue, round 
‘ which the cells arrange themselves in a regular wall. This 
manner of forming the flagellated chambers is obviously only 
an extension of the second method, in that very many of the 
formative cells early break up into masses of fine cells. 
In the larva during the course of attachment one or two 
flagellated chambers sometimes make their appearance, as in 
Pl. XVI, Fig. 36, 7. .c:; but the details of the formation of 
such chambers were not worked out. 
