No. 3-] DEVELOPMEyT OF MARINE SPONGES. 347 



Hircinia there is the same indication as in Tedanione, that the 

 mesoderm cells bring food to the growing egg. Until segmenta- 

 tion begins the egg is thickly surrounded by mesoderm cells, PL 

 XXIV, Figs. 116 and 119, which have large bodies full of fine 

 yolk granules. Amongst these cells are scattered a few with 

 very coarsely granular bodies. Fig. 119. The mesoderm cells 

 are applied so closely to the follicle that the latter cannot be 

 distinguished as a definite layer. All that can be said is that 

 at this time the egg is surrounded by closely packed cells, 

 arranged irregularly in two or three strata, the inner stratum 

 doubtless representing the follicle of the very young egg, 

 while the outer strata consist of mesoderm cells which have 

 applied themselves to the follicle. The inner stratum of cells 

 is, as may be seen in Fig. 116, often very irregular, and there 

 are certain indications that the follicular cells are sometimes 

 pinched off and engulfed by the egg — notice the very protu- 

 berant rounded cells projecting into the substance of the egg. 

 Whatever be the precise manner in which food is conveyed 

 from the surrounding cells to the egg, it seems pretty certain 

 that food is so conveyed, and that this is the object of the 

 migration of so many mesoderm cells to the immediate neigh- 

 borhood of the growing egg. The egg at the time when it 

 reaches full size is still surrounded by several strata of cells, 

 Fig. 119. The inner stratum, however, soon becomes trans- 

 formed into a follicular membrane, consisting of flattened but 

 still rounded and protuberant elements, PI. XXIV, Fig. 120. 

 The cells of the outer strata gradually %vander away, leaving 

 the mesoderm round the segmenting egg not more abundantly 

 supplied with cellular elements than is the mesoderm in most 

 parts of the sponge. 



The ripe egg of Hircinia is closely packed with yolk spheres 

 of a large size, Fig. 119, which make their appearance in the 

 developing egg-cell after a fashion essentially similar to that 

 already described for Tedanione. In the finely granular body 

 of the very young egg-cell, yolk spheres appear which are at 

 first small. Figs. 115, 115', but which gradually increase in 

 size, becoming at the same time more closely packed. In the 

 egg shown in Fig. 116, the bulk of the yolk is still composed 



