STRUCTURE OF THE LARVA OF SPONGILLA LAOUSTRIS. 417 



of such evidence it seems impossible to doubt that a great 

 number of the bodies contained in the plasmodial aggregations 

 give rise to the nuclei of the collar-cells. 



Fig. 17 illustrates the commencement of the breaking up 

 of the plasmodial aggregations, and the change which takes 

 place in the characters of the nuclei of the flagellated cells. 

 Both of these changes are seen to have advanced still further 

 in figs. 18, 18 a, and 18 b, in which, however, there is no sign 

 as yet of a flagellated chamber. In figs. 19 and 19 a, which 

 still lack flagellated chambers, the plasmodial aggregations 

 have disappeared as completely as if they had melted away. A 

 stage is therefore produced in which the cells present a syncytial 

 arrangement. In fig. 20 a stage is illustrated in which the 

 nuclei of the flagellated cells are becoming arranged in rings 

 round cavities which in some cases are more or less circular 

 in form, but the general arrangement is still syncytial. Fig. 

 21 represents a further change. The cytoplasm, surrounding 

 the nuclei arranged in rings in the stage represented in fig. 20, 

 becomes divided into masses corresponding to the nuclei, and 

 the individual collar-cells provided with a collar and flagellum 

 make their appearance. 



The changing nuclei described above must not be confused 

 with the yolk bodies which exist inside the cells with vesicular 

 nuclei, and which become smaller stage by stage, while the 

 nuclei of the flagellated cells, on the contrary, become larger. 

 The number of yolk bodies found in the cells with vesicular 

 nuclei varies considerably, according as the larva fixed at an 

 early stage of development or a late one. In fig. 26 the yolk 

 bodies are coloured blue, while the small nuclei are red. In 

 figs. 18, 19, 20, and 21 the distinction between yolk bodies and 

 small nuclei is such that it is impossible to mistake the one for 

 the other. The important point for our purpose, however, is 

 that the nuclei of the flagellated cells and the yolk bodies exist 

 side by side in the plasmodial aggregations, the former giving 

 rise to the nuclei of the collar-cells, while the latter are used up 

 during the development as food material. 



To sum up, the special features of the pupa derived from the 



