t 



EXPLANATION OF PLATE XVIL— continued. 



flap. mu. Muscular-cells passing from body wall to centrally placed gas- 

 trula-endoderm. sA. Shell-rising up as a watch-glass, n g. Nerve-ganglion. 

 t. Tentacle, v. Velum. 



Fig. 9. — a, b, c. d. — Pour conditions of cells of the gastrula-endoderm 

 from embryos a little younger than that of Fig. 8. 



Pig. 10. — A similar embryo to that of fig. 7. Letters as before. 



Pig. 1L — A small embryo (about -^ inch long) from a mass containing 

 embryos similar to those of figs. 7 and 10. It is irregularly developed and 

 has the shell-gland occupied by a bright highly-refracting plug, is. f. Poot. 

 ph. Pharynx, v. Velum, mf. Mantle-flap. sh. Shell, is. Plug of the 

 shell-gland, p i. Pedicle of invagination (rectum). 



Pig. 12. — Shell-gland with plug is, and external shell es, of the same 

 embryo. 



Pig. 13. — SheU-gland of another embryo and circular shell-area covered 

 by the disc-like shell. 



Pig. 14. — SheU-gland and widely open mouth of the same from another 

 embryo. 



Pig. 15. — Optical section of the central portion of the shell-secreting area 

 at a latep stage (viz. that of fig. 8), showing the last vestige of the shell- 

 gland as a small depression. 



Pig. 16. — Optical section of shell-secreting area of an embryo, similar to 

 that of fig. 17, showing the superficial columnar and deeper fusiform cells, 

 and the delicate shell, sh. 



Fig. 17. — An embryo younger than that] of fig. 5, considerably younger 

 than that of fig. 8, compressed and treated with osmic acid. Long diameter, 

 as thus seen, about -^ inch. /. Poot. ph. Pharynx, v. Velum, ng. 

 Nerve-ganglion, t g e. Tunic of fusiform cells covering in the gastrula. 

 endoderm. sh. Shell, ss. Shell gland, pi. Pedicle of invagination. 

 g s. Right-hand cavity of the bilobed gastrula-stomach. Observe the 

 fusiform cells passing from the body-wall to be connected with this. 



Fig. 18. — An embryo of nearly the same age and size as that of fig. 10, 

 treated with osmic acid. v. Velum. /, Bilobed foot. ph. Pharynx. 

 n g. Nerve-ganglion, h. Heart, mf. Mantle flap. n. Kidney formed as 

 an in-pushing from the reflected flap of the mantle, m. Muscular fibres 

 attached to the bend of the intestine, i. Intestine, g. Problematical mass 

 of cells lying close to the kidney. I. Lung-chamber, c r. Csecal terminal 

 tion of the intestine, r m. Retractor (?) muscle. 



Pig. 19. — Cells of the body- wall and gastrula-endoderm connected by 

 processes of the latter, from an embryo of the same age as that of fig. 4. 



Fig. 20. — Recurved and csecal termination of the rectum developed from 



