STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 399 



cells with granular nuclei will only spread gradually. The 

 flagellated layer will in consequence remain for a long time on 

 the upper surface, and immigrate from it slowly, the cells pass- 

 ing in one by one. This is the natural result of the breaking 

 out of the cells with granular nuclei at one point rather than at 

 many points. The internal pressure has thereby been decreased 

 to such an extent that the resistance of the flagellated layer is 

 enough to prevent the cells with granular nuclei spreading 

 over the upper surface until the pupa,^ by its expansion, has 

 decreased the cohesion of the flagellated layer. 



These two processes go on at the same time. While the flagel- 

 lated cells are passing in, the cells with granular nuclei struggle 

 to the exterior. The process is, apparently, a reciprocal one in 

 which both classes of cells take an active part. The reversion 

 of the layers does not take place so quickly on the upper as it 

 does on the lower surface, and this is especially true of type C. 

 There is also a slight difference between the cells with granular 

 nuclei situated at the upper and lower surfaces respectively, 

 the nuclei of those in the former position being smaller than 

 those of the cells in the latter. The cells with granular nuclei, 

 after passing through the layer of flagellated cells and arriving 

 at the surface, become flattened out, their edges meeting one 

 another. In this way a continuous layer of cells is formed on 

 both upper and lower surfaces, derived from those cells which 

 in the free-swimming larva are situated below the flagellated 

 layer ; that is to say, they formed originally a part of the 

 " inner mass." 



Owing to the large size of the nuclei of those cells which 

 pass to the lower surface, a most satisfactory and conclusive 

 proof is obtained that the flattened epithelium of the young 

 sponge is formed from cells which once lay in the interior 

 of the larva, and not from the flagellated cells at the 

 surface. The nuclei of the latter are small, and whatever 

 other change they may undergo at this stage, it is certain that 



1 This term is used by Mr. E. A. Mincbin to indicate that stage in the 

 development which occurs between actual fixation and the appearance of the 

 dermal pores and osculum. 



