STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 469 



nucleus at the centre, together with smaller nuclei and yolk bodies nearer the 

 surface. The yolk bodies are almost indistinguishable from the small nuclei 

 of the flagellated cells during this stage. Note also the large granular nuclei 

 situated on the lower surface, which owing to their large size cannot possibly 

 be derived from the nuclei of the flagellated cells. 



Pig. 17. — X 1000. Portion of a section of a pupa slightly more advanced 

 than that represented in Pig. 16, derived from tjpe D. The nuclei of the 

 flagellated cells in the plasmodial aggregations are changing in character ; that 

 marked a has become larger in size, and the chromatin in it is evenly dis- 

 tributed ; in that labelled b strands or thick threads appear; while in tliose 

 marked d the ultimate structure of the nuclei of the collar-cells has been 

 practically attained. All these four nuclei are still well within a perfectly 

 definite plasmodial aggregation, proving in a most satisfactory way that the 

 nuclei of the flagellated cells become the nuclei of the collar-cells, since the 

 four nuclei in question are hardly distinguishable from those marked e, which 

 are undoubtedly the nuclei of cells which later on become collar-cells. 



Pigs. 18, 18 a, 18 b. — x 1000. Section of a pupa which is slightly more 

 advanced than the one represented in Pig. 17. The plasmodial aggregations 

 {pi. a.) are losing their sharp outline and are becoming gradually united with 

 one another. The small nuclei of the flagellated cells, which in Pigs. 16 and 

 16 a are almost indistinguishable from the yolk bodies, are in this stage easily 

 made out, owing to their loose internal structure, and the thick strands of 

 chromatin which appear in them. 



Pig. 18 represents a portion near the margin of the section, and though 

 there are several plasmodial aggregations it does not contain a single vesicular 

 nucleus. In Pig. 18 a, which is further in, an occasional vesicular nucleus 

 is seen ; and in Fig. 18 ^, which is drawn from near the middle of the section, 

 there are several. The cells with vesicular nuclei, therefore, are found at the 

 centre of the young sponge, which means that thuse near the margin have 

 been already transformed into cells with granular nuclei, which very likely 

 become pore-cells. 



Pigs. 19, 19«. — X 1000. Portions of a section of an individual which 

 is slightly more advanced than that drawn in Pig. 18, derived from 

 type D. The plasmodial aggregations of cells have completely lost their 

 individuality and have become indistinguishable from one another. In 

 Pig. 19 a the cytoplasm is arranging itself in the form of a ring, the 

 cavity of which represents that of the future chamber, and the nuclei en- 

 closed in it are those of the future collar-cells. In Pig. 19 large spaces have 

 appeared inside the young individual. The upper one (S. C.) represents the 

 subdermal cavity, which has been produced through the pushing out of the 

 dermal membrane {D.) by the outgrowing spicules. 



