STRUOTUEE OF THE LARVA OP SPONGILLA LACUSTRIS. 429 



originate from the same cells, they are different in their mode 

 of origin as well as in their destiny. 



The above conclusion may be further strengthened by the 

 analogy of the early development of the larva while still within 

 the maternal follicle. At the close of segmentation the young 

 embryo consists only of cells with vesicular nuclei. Subse- 

 quently the outer layer of these divides up to produce the 

 flagellated cells, while some of the cells more deeply situated 

 give rise to cells with granular nuclei. In this way the youngest 

 free-swimming larva, described in this paper as type A, origi- 

 nates. In its development, cells which were quite similar in 

 character gave rise to two different classes of cells, namely, 

 flagellated cells at the surface and cells with granular nuclei 

 within. In tlie later stages of development, that is in the pro- 

 duction of types B and C, these two processes seem to be going 

 on, but instead of going on at the surface they go on in the 

 interior of the solid part of the inner mass. The formation of 

 the cell groups in the interior corresponds to the formation, 

 during the earlier stages, of flagellated cells at the surface, 

 and is, in fact, a continuation of the same process, just as the 

 formation of cells with granular nuclei deep in the interior of 

 the inner mass is a continuation of the same process that gave 

 rise to similar cells underneath the flagellated layer. 



If this view be correct, a process of differentiation is con- 

 tinually going on, producing, on the one hand, cells with nuclei 

 exactly similar in their character to those of the flagellated 

 cells ; and, on the other hand, adding to the number of the cells 

 with granular nuclei. It might be argued that these cells do 

 not develop the flagella characteristic of the flagellated cells, 

 and cannot, therefore, be homologous with them, or even belong 

 to the same class of cells. The reply to this argument is that 

 they do produce flagella, and that later on they develop a 

 collar. As collar-cells their shape is not very diff'erent from 

 that of the flagellated cells, the nuclei in both being situated 

 at the base of the cell. 



The conclusion adopted, after all these arguments for and 

 against, is that the flagellated cells at the surface, and the cell 



