412 EIOHARD EVANS. 



no longer independent. Fully formed plasmodial aggregations 

 are found to exist, in addition to chamber-like rings of cells 

 derived from the '' cell groups " of the larva. The nuclei of the 

 plasmodial aggregations, however, in a short time emancipate 

 themselves, and the cells consequently, becoming free and 

 sharply individualised, give rise to the collar-cells of a new 

 series of flagellated chambers. The collar-cells of the sponge 

 are therefore developed, on the one hand, from the cells of the 

 inner mass, and, on the other hand, from the flagellated cells. 

 Tn fig. 28 two flagellated chambers are represented ; in one 

 of them the cells are perfectly separate and independent, 

 with the nuclei all alike, and possessing the same structure 

 as the nuclei both of the collar-cells — i. e. chamber cells, 

 for they have not as yet developed collars, represented in 

 figs. 24 b, 31, and 31 a — and of the cells which have already 

 developed collars and flagella as seen in fig. 29 a. In the 

 other chamber in the same figure the cells are not so dis- 

 tinct, and the nuclei are much smaller and more irregular in 

 size. In fact, both the arrangement of the cells and the size 

 and structure of the nuclei resemble those of the flagellated 

 chambers represented in figs. 19, 19 a, 20, and 21, which are 

 drawn from specimens developed from type D, in which, how- 

 ever, flagellated chambers derived from '' cell groups " are 

 practically absent. It is evident that the cells of the lower 

 chamber in fig. 28 have emancipated themselves from one or 

 more plasmodial aggregations, which they formed in combina- 

 tion with the large cells with vesicular nuclei, situated close to 

 the chamber in question. With regard to these cells with 

 vesicular nuclei, it may be pointed out that those in the 

 immediate neighbourhood contain very few yolk bodies, which 

 at this stage are of necessity much reduced in size, while 

 those further off* contain more yolk bodies. This fact points 

 to the view that the yolk bodies in these cells have been 

 almost completely used up by the flagellated cells which grouped 

 themselves round the cells with vesicular nuclei to form the 

 plasmodial aggregation. As has been stated above, the nuclei 

 of the flagellated cells in the plasmodial stage can be distin- 



