STRUCTURE OP THE LARVA OF SPONGILLA LAOUSTRIS. 475 



Fig. 36 c this stage has been reached, the nucleus being similar to that of any 

 flat epithelial cell. 



Figs. 37 a, b. — X 1000. Microscleres in their epithelial scleroblasts, 

 completely enclosed by the cells which secrete them. 



Fig. 37 a represents spicules from a larva belonging to type C, preserved 

 immediately after fixation, i. e. while the flagellated layer was still complete at 

 the upper surface. 



Fig. 37 b represents two cells of the flat epithelium, one of which contains 

 a microsclere, drawn from a somewhat advanced young sponge. 



Figs. 38, 39. — x 1000. These figures represent the cellular elements of 

 a larva which was preserved in osmic acid, stained in picro-carmine, and 

 mounted in glycerine. Owing to the maceration that had taken place 

 during fixation and staining it fell immediately into pieces, and the cells 

 separated. 



Figs. 38 a — d represent the flagellated cells, and show small granules 

 which were not seen by the aid of any other method of preparation. 



Fig. 39 shows a portion of the flagellated layer from within, and the 

 granules are seen in all the cells. 



Figs. 38 ff, b show the difference in length which obtains between the 

 flagellated cells, c the flagellum passing down to the nucleus, while in d it 

 could not be traced any further than a small and irregularly shaped granule 

 situated about halfway down to the nucleus. 



The cells e and / are extremely irregular in shape, while the outline of the 

 cell g is much more definite. The cells e and f contain far less granules than 

 the cell g, a fact which may have some relation to their shape, the cell g 

 being rendered more inert than the others by the granules. 



The cells f and g, which are provided with a vesicular nucleus, contain a 

 large nutritive vacuole, blackened by the osmic acid. The cell g contains 

 several large yolk bodies {y. b.), while the cell e has only one. 



All these cells contain small refringent granules, probably reduced yolk 

 bodies, though they are not shown in the flagellated cells of the larva by 

 any other method of preservation. Compare Fig. 29 a. 



Figs. 40 a — g'. — x 1000. Development of the cell groups characteristic 

 of the larva of type B. For description see Appendix B, on pp. 425 — 430. 



Fig. 41 «. — x 1000. Figure showing the difference of structure between 

 the vesicular nuclei ; one, with a few but large granules, is in the same state 

 as the cell a of Fig. 40 ; the other represents the structure of an ordinary 

 vesicular nucleus. 



Fig. 415. — X 1000. Two cells which, to judge from the number of yolk 

 bodies contained in them, originally had vesicular nuclei. The nucleus, 

 however, has become fragmented, and is represented by a number of small 

 granules situated in the centre of the cells. The condition of the nucleus 



VOL. 42, PART 4. NEW SERIES. K5 



