No. 3] DEVELOPMENT OF MARINE SPONGES. 313 



in size they become arranged in a single layer, an. f. c", Fig. 40. 

 The surface of the flagellated chamber so formed gradually 

 becomes smooth, and its shape, which may be of almost any 

 character {aii.f. c", an. f. c.'", Fig. 41, and an. 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. e.g., PL XVIII, Fig. 

 57)> gi^''"g ""'se 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, /. 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 XVII, Fig. 36, /. c. ; but the details of the formation of 

 such chambers were not worked out. 



