STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 391 



that of the nuclei of the flagellated cells. Further^ they 

 possess the same structure as the nuclei of the latter cells — that 

 is, a few granules situated at the nodes of a coarse network of 

 threads and a thick nuclear membrane. There is no doubt but 

 that these cells belong to the same class as those of the cell 

 groups found in types B and C, and their presence, however 

 few they may be, accounts for the existence of an occasional 

 chamber in a larva of type D, immediately after fixation. 



The cells with vesicular nuclei present much the same 

 characters and general distribution as they do in type C. 

 They are both less numerous and smaller than tliey are in 

 types A and B. The number of yolk bodies which they 

 contain is much smaller than in types A and B. The 

 nutritive vacuole of course is a constant feature of the cell, 

 with a vesicular nucleus in all stages, but it appears to 

 decrease in magnitude simultaneously with the decrease in 

 size of the cell. The inner mass, therefore, in this larva 

 consists of a fair number of cells both with granular and with 

 vesicular nuclei, together with a few of the elements of the cell 

 groups, in contrast with the large number of them found in 

 types B and C (figs. 6 and 6 a). 



General Remarks upon the Relationship of the 

 Larvse. — The author's views with regard to the relationship 

 of the four larval types already described have been stated 

 above (p. 369). It only remains to discuss these relations in 

 greater detail. 



There can be no doubt that type A is the youngest larva of 

 the four types described above. The fact that the cells with 

 vesicular nuclei are here relatively more numerous than they 

 are in any other type supports this view, because they are the 

 most primitive cells of the larva. They retain, in fact, in 

 type A almost all their blastomeric characters, which in the 

 other types they gradually lose, so far as the contents of the 

 cells are concerned, though not perhaps in the physiological 

 sense. 



Histogenesis seems to advance or to be retarded in separate 

 regions of the larva at different times. At the close of 



