STRUCTURE OP THE LARVA OP SPONGILLA LACUSTRIS. 467 



flagellum. The yolk bodies (y. b.) are far less numerous, though always 

 present, as will be shown in more highly magnified figures. The nutritive 

 vacuoles are smaller, owing to the fact that the cells with vesicular nuclei 

 which contain them are smaller. This figure was produced in the same way 

 as Fig. 9. 



Tig. 11 a. — X 1000. Portion of a section at the junction of the larval 

 cavity with the posterior part of the inner mass, from the same larva as 

 Fig. 11. The cell groups, which are very numerous, are arranged in chamber- 

 like rings. The cells with granular nuclei (c. g. n.) lying towards the larval 

 cavity are highly flattened, forming a lining to it. The nutritive vacuoles 

 {b. n. V.) are blackened by the action of the preserving reagents, and the yolk 

 bodies are few and reduced iu size. 



Fig. 12. — X 1000. A group of small nuclei such as is characteristic of 

 type B, from a larva which in other respects possesses all the characters of 

 type C. The difference between the small nuclei and the granular nuclei of 

 the cells which surround them is most striking, and their presence in this 

 stage of development in type C points to their production being a process 

 which goes on continuously. 



Figs. 13, 13 «, IZb. — X 1000. Sections of a larva of type C to show 

 that the larvae contain yolk granules (y. b.) even in the oldest free-swimming 

 stage. Besides those in the cells with vesicular nuclei, even the cells with 

 granular nuclei and cell groups contain a few of them, a proof that the 

 number of granules in type C is a variable quantity. 



In Fig. 13 there is a ring-like group of cells, among which a very interesting 

 cell occurs, showing a stage of the nucleus preparatory to division. The 

 nucleus {n. g. c.) has travelled from the base of the cell to the side, and has 

 lost its usual structure, and has an irregular blotched form. 



Fig, 14. — X 1000. Section through the side of the cavity of a larva of 

 type C when on the point of becoming fixed ; showing the region where the 

 cells with granular nuclei first make their way out, and the flagellated cells 

 immigrate. The structure of the small nuclei {n.fl. c.) is in some cases already 

 beginning to change. They become smaller in size, though tiie actual 

 quantity of chromatin seems to increase. The linin threads thicken, and the 

 chromatin shows a tendency to become aggregated into one or more irregular 

 patches, lying close to the nuclear membrane. Owing to these changes it 

 is already difficult to distinguish some of the small nuclei from the reduced 

 yolk bodies, though they are easily distinguished from those that are not 

 reduced. 



Fig. 14«. — X 1000. Two cells from the same larva as Fig. 14. They 

 show how the cells with granular nuclei flatten out to enclose the rudiment 

 of an exhalant canal in the free-swimming larva, and should be specially com- 

 pared with the cell groups found in the same type of larva. 



