910 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [20] 



EXPLANATION OF THE PLATES. 



Fig. 1. Egg of the oyster, with the germinal vesicle and spot visible within the 

 vitellus. 



Fig. 2. Beginning of development ; the germinal vesicle has become invisible, and 

 the polar globules make their appearance. 



Fig. 3. First stage of segmentation ; the egg is divided into two spheres of unequal 

 size (animal sphere and vegetative sphere). 



Fig. 4. More advanced stage, the egg being divided into four spherules. 



Fig. 5. Stage of development where the egg shows one large vegetatiA'e sphere and 

 several animal spheres. 



Fig. 6. More advanced stage, seen from above, where the vegetative sphere is di- 

 vided into two spherules. 



Fig. 7. Embryo, side view, at the beginning of invagination (gastrula). 



Fig. 8. Embryo more developed, optic section, with invagination of the entoderm 

 and begiuniug of the preconchylian gland ; ee, ectoderm ; en, entoderm ; o, blastopore ; 

 sk, ]»reconchylian gland. 



Fig. 9. Embryo a little older, side view ; v, pediform appendix; the other letters 

 as in preceding figure. 



Fig. 10. Same stage, optic section; me, mesoderm; d, primary intestine. 



Fig. 11. Embryo a day older, front view, with the primary mouth opening. 



Fig. 1"2. The same embryo, optic section. 



Fig. 13. Embryo a day older, with a wreath of vibratile cilia, a stomachic cavity, 

 and a beginning of a shell, s. 



Fig. 14. More advanced stage, side view, with shell more developed. 



Fig. 15. Larva still more developed, with velum, and the beginning of the cephalic 

 plate (scheiteJplatte); a, anus; e. intestine; m, stomach; si, esophagus; kp, cephalic 

 plate. 



Fig. 16. Larva a little older, with a double pre-oral wreath of vibratile cilia, a 

 cephalic plate, hepatic pouch, and muscles; ds, longitudinal dorsal muscle; vs, longi- 

 tudinal ventral muscle; sj), adductor muscle ; /, hejiatic pouch; mh, mantle cavity ; 

 the other letters as above. 



Fig. 17. The velum or rotary disk, with the double row of ciliated cells, seen ob- 

 liquely from above. 



Fig. 18. Larva as it attaches itself, in an almost vertical position. 



Fig. 19. Little oyster, about 7 days old ; the height of the primary homogeneous shell 

 is 0.24 millimeter, that of the secondary part, composed of prisms, is 0.15 millimeter; 

 the beginning of the branchiae and the adductor muscle are visible. 



Fig. 20. Fragment of the edge of the preceding shell, to show how the calcareous 

 prisms are formed. 



Fig. 21. Fragment of the lower valve. 



