ON THE DEVELOPMENT OF THE POND-SNAIL. 389 



finally divides into two clear specks, of which one is found in 

 each half of the cleft yelk-mass. These clear specks are 

 undoubtedly true nuclei of the cleavage spheres, but no con- 

 nection between them and the germinal vesicle could be 

 demonstrated. These nuclei divide again, and at the same 

 time the yelk is seen to fall into four masses by the formation 

 of a second cleft. According to Lereboullet all the cleavage- 

 spheres fuse themselves into a homogeneous mass before each 

 new segmentation every time, and then separate again and 

 commence the process of further cleavage. According toQuatre- 

 fages the same thing occurs in the annelid Sabellaria. From 

 the midst of four cleavage-spheres so formed and lying m one 

 plane arises now a clear vesicle, which quickly divides into 

 four small nucleated spheres, and soon both the four large and 

 four small spheres — always preceded by their nuclei — again 

 divide, so that the egg now consists of sixteen nucleated 

 cleavage-spheres (b) . The large spheres overgrow now with 

 their progeny the smaller, and we have at last a spherical 

 mass, which consists externally of large, internally of small 

 cleavage-spheres, accordingly exactly the reverse of what 

 occurs with the cleavage-spheres of the Opisthobranchiata 

 and Prosobranchiata (c). Finally, however, the segmented 

 effs consists of large nucleated cells '02 to "025 mm. in 

 diameter, which do not as yet present any cell-membrane. 



" At the end of the second day modifications of this cell- 

 mass are seen. On one side the cell-mass hollows itself 

 out, then flattens itself, and at the same time the in-sinking 

 narrows its area, so that the mass presents a cavity within 

 and a narrow emarginated opening leading into it. These 

 are the alimentary cavity and the mouth (d) ; the outer cells 

 are still larger than the inner ones. Beneath the mouth the 

 body now flattens itself out and forms a process — the foot, and 

 the embryo begins now its well-known and — since the time 

 of Leuwenhoeck — celebrated slow movements of rotation. 

 This rotation must be ascribed to cilia ; but they are so fine 

 that they have not been seen in Limiiceus, although one may 

 suppose by analogy that they exist especially on the foot (e). 

 The foot grows more and more prominent, and the body 

 becomes partially embraced by an annular ridge, in which the 

 later mantle-margin is soon recognised (f). From behind 

 now — over against the mouth — a new in-sinking is formed, 

 anus and rectum, which grows up against the primitive 

 aKmentary cavity, and finally unites with it (g). The ali- 

 mentary tract now becomes hollowed out, and in its neigh- 

 bourhood large yelk-spheres are formed, the first rudiments 

 of the liver (h). 



