JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XXIV, 



Illustrating Professor Lankester's Remarks on the Shell- 

 gland of Cyclas and the Planula of Limnseus. 



Fig. 1. — Embrvo of Ci/clas cornea, showini^ both sliell-js^land and byssus- 

 gland. .S/^.-(?/. Siiell-gla'ud. 5y.-(?/. Byssus-Gland. P/i. Pharynx, i-^ Foot 

 M. P. Rectal peduncle. 



Fig. 2. — An embryo of Cyclas cornea somewhat further advanced in 

 development. The shell-gland has now flattened out, and there is evidence 

 of a delicate secretion on its surface, the single primordial shell. Letters 

 as in fig. 1. 



In neitlier of these figures is any attempt made to give histological detail ; 

 but the drawing is intended solely to represent the appearances in a half- 

 diagramatic style which are seen when the embryo is living, and examined 

 with Hartnack's obj. 8, ocular 4. 



Fig. 3. — Morula of Limnseus seen from the antiklastic pole. Four larger 

 cells are seen, between which, at the periphery, are the tips of four en- 

 croaching smaller cells. 



Fig. 4. — Lateral view of a morula of Limnfeus of the same age. The 

 large size of the cells at the autiklastic pole is obvious. D. C. Directive 

 corpuscles. Ch. Chorion. 



Fig. 5. — A morula of Limnseus, a little more advanced than that of fig. 3, 

 with eight in place of four encroaching cells, viewed from the antiklastic 

 pole. 



Fig. 6. — A similar morula seen from the side. D. C. Directive corpuscles. 

 Ch. Chorion. 



Fig. 7. — Optical section through a morula a little more advanced than 

 that of fig. 6, showing the formation of a cavity at the klastic pole, and the 

 long processes (mesoblastic ?) of the pale cells of the klastic hemisphere. 

 D. C. Directive corpuscles. Ch. Chorion. 



Fig. 8. — A surface view of the same embyro. 



FiG.9. — Outline of the invaginate planula or gastrula of Limnseus, to show 

 the elongated character of the blastopore. Or. Oral end of the blastopore. 

 Rect. Rectal end. 



