372 RIOHAED EVANS. 



and develop collars and flagella (figs. 16 — 22, fi. c. = c. c). 

 In the meantime the cells with granular nuclei which are still 

 in the interior are arranging themselves to form alining to the 

 cavities which have appeared in the syncytium-like parenchyma 

 of the young sponge. The spaces later on become the sub- 

 dermal cavity, the inhalant and exhalant canal systems, and 

 the gastral cavity ; in short, all the cavities of the sponge, 

 other than the chambers, are lined by cells with granular 

 nuclei. This description of the fate of the flagellated cells 

 applies equally to the larval types C and D. The history of the 

 flagellated cells, however, is more easily made out in the type 

 D, owing to the almost complete absence of the small cells 

 which compose the cell groups, than in type C, where the cells 

 in question are numerous. 



In the type C, flagellated chambers are in many cases fully 

 developed at the time of fixation, possessing cells which are 

 adorned with collars and flagella (figs. 7 and 7 a). These cells 

 are found in groups in type B, and are seen to possess at a 

 certain stage a nucleus so exactly like that of the flagellated 

 cells as to be indistinguishable from it (figs. 5 c and 9 a and b). 

 During the change from type B into type C the nucleus alters 

 in character, and assumes the definitive structure of the nucleus 

 of the collar-cells. Hence in the complete history of the 

 flagellated cells on the one hand, and of the small cells in the 

 ''cell groups" on the other, we have two stages in which their 

 nuclei are exactly like one another. First, the nuclei of the 

 small cells of the groups at the time when the cytoplasm is as 

 yet incompletely divided up are exactly like the nuclei of the 

 flagellated cells, so long as the latter retain their position in 

 the superficial layer of the larva. Secondly, they resemble one 

 another so much as to be indistinguishable when both have 

 attained the definitive structure of the nucleus of the collar-cell. 

 Though these two developmental stages correspond exactly, 

 the intermediate conditions through which they pass are very 

 different ; the nuclei of the cell groups pass gradually from one 

 stage into the other without any interruption, while those of 

 the flagellated cells pass through a most extraordinary series of 



