JOURNAL OF MICEOSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XVI, 



Illustrating Mr. Ray Lankester's Memoir on the Develop- 

 ment of the Pond-snail. 



Fig. 1. — An egg after the formatiou of the first cleavage groove. Two 

 Riehtungsbljiscben (R.) are seen. Nat. size of the egg = ^^ inch. 



Fig. 2. — An egg after division into four cleavage-masses, three of whicli 

 are seen, and the Riehtungsblaschen (R.). Nat. size -^^ inch (long measure- 

 ment). 



Fig. 3. — The same quadripartite egg seen from below. 



Fig. 4. — An egg of a later stage in which smaller cleavage-spheres have 

 made their appearance at one pole. A Riehtungsblaschen (R.) is seen 

 attached between the four larger cleavage- spheres. 



Fig. 5. — The same egg seen from above. 



Fig. 6. — The same egg seen from below. 



Fig. 7. — A later stage. At the pole ff m the gastrula-invagination is now 

 commencing. At R the Riehtungsblaschen, entangled in the discarded 

 vitelline (?) membrane, is seen. 



Figs. 8 — 12. — Various views of the Gastrula of Lymnceus. Natural 

 size = yIo inch. The appearance varies according to the position which 

 is assumed. Fig. 9 gives a surface view as seen by reflected, light. Fig. 

 10. The same specimen seen by transmitted light. Fig. 11. Another speci- 

 men, two thirds profile view; gm,\% the gastrula-mouth or orifice of invagi- 

 nation. 



Fig. 13. — The early phase of the Trochosphere with large lateral lobes (the 

 figure is turned sideways), m. Commencing formation of the permanent 

 mouth. Long measui'ement of this specimen ^lo '^ch. 



Fig. 14. — Trochosphere with ciliated annular ridge and commencing 

 mouth, m. The large cells of the Gastrula-endoderm are seen to be in 

 connection with the body wall by means of delicate processes. Longest 

 measurement of this specimen = j^ inch. 



Fig. 15. — a, b, c, d. Successive outlines presented by such a trochosphere 

 as that in the preceding figure during rotation in the autero-posterior direc- 

 tion. The small prominence seen in b, and also in fig. 13, in a similar posi- 

 tion, is probably the first indication of the foot. 



