BULLETIN OF THE UNITED STATES FISH COMMISSION. 167 



12. 



Explanation of the Plate. 



Fig. 1. — Segmented egg showing the large vegetative cell below, and the numerous 

 animal cells above. 



Fig. 2. — Older stage, seen from above, showing the two entodermal cells shimmering 

 through from below. 



Fig. 3. — Embryo seen from the side, showing the commencement of the gastrular 

 invagination. 



Fig. 4. — More advanced stage, seen in optic section, showing the entodermal invagi- 

 nation and the commencement of the shell gland ; ec, ectoderm ; en, ento- 

 derm ; o, gastrula mouth — blastopore ; sk, shell gland. 



Fig. 5. — Still older embryo, seen from the side ; v, foot ; the other letters with the same 

 signification as before. 



Fig. 6. — The same stage soen in optic section ; me, mesoderm ; f7, gastrula cavity or 

 archenteron. 



Fig. 7.— Embryo one day older, seen from the anterior end, showing the triangular 

 opening of the blastopore. 



Fig. 8. — Tbe same stage in optic section. 



Fig. 9. — Embryo one day older, seen in section, with ciliary crown (vellum), stomach 

 cavity, and rudiment of the shell developed ; s, shell. 



FlG. 10. — A more advanced stage seen from the side, with tbe shell further developed. 



FlG. 11. — Larva still more advanced, with velar disk or area developing, the central 

 thickening of which, tp, is apparent; a, anus; e, rectal end of intestine ; 

 m, stomach; «7, o-sophagus ; tp, rudiment of supraceBophageal ganglion. 



FlG. 12. — More advanced larva with a double prooral girdle of cilia, hepatic sac, mus- 

 cles, and rudiment of supraoesophageal ganglion developed; ds, dorsal 

 pallial muscle ; rs, ventral pallial muscle ; sp, adductor muscle ; I, hepatic 

 sac or diverticulum of the stomach ; mh, mantle cavity. The remaining 

 letters have the same signification as in x^revious figures. 



