410 RICHARD EVANS. 



flagellated chambers (figs. 29 and 29 a), in which the individual 

 cells are provided with a collar and a flagellum (fig. 29 c), 

 resembling in every respect those of the collar-cells of the adult 

 sponge. There can be no reasonable doubt, therefore, as to the 

 origin of the chambers in question from the cell groups of 

 types B and C. Inasmuch as the cell groups found in type 

 B are developed from the cells of the inner mass, that is, 

 from the cells with vesicular nuclei ; and since, further, these 

 chambers are produced from the cell groups ; it follows that 

 the flagellated chambers found in the young pupa derived from 

 type C, and represented in figs. 29 and 29 a, are produced by 

 the multiplication of the cells of the inner mass, and not from 

 the flagellated cells which migrated into the interior, and 

 which have as yet, even at the lower surface, only gone so far 

 as to form plasmodial aggregations (fig. 29 b). These facts 

 furnish a clear proof that cells of the inner mass are capable 

 of giving rise to collar-cells, and that the flagellated chambers 

 do actually so arise in the development. The cells arranged 

 in groups surrounding spherical cavities in the larva do not 

 flatten out to form the canals of the sponge. On the other 

 hand, it has been shown that the cells which in the larva become 

 flattened to form the lining of the lacunar spaces, belong to 

 an entirely different class of cells, i.e. the class which has been 

 described as consisting of cells with granular nuclei, from 

 which arise the epithelial membranes of the sponge in general. 



Having traced the origin of the flagellated chambers from 

 cells of the inner mass, it remains to inquire what happens to 

 the flagellated cells after they have entered the plasmodial 

 aggregations in those pupae in which the cells of the inner 

 mass do indubitably take part in the formation of the flagel- 

 lated chambers. 



It has already been pointed out that the flagellated cells 

 migrate into the interior and form plasmodial aggregations 

 (figs. 29, 29 6, 24 6, and 25), though they migrate far 

 more slowly from the upper than from the lower surface. lu 

 figs, 30 and 30 a the flagellated layer, which has not com- 

 pletely disappeared from the surface even at this stage, 



