KEY TO PLATE I 



All the figures of this plate arc drawn as accurately as they admit of, 

 to the same scale of magnification, viz., 150 diameters, so that the relative 

 sizes of different stages may be seen at a glance. They are also oriented 

 in the same position throughout, with the dorsal margin upwards and the 

 anterior end to the observer's right, i.e., they lie on the left side like the 

 attached oyster. 



Fig. 1. Oyster's ovum. Oc. 5, obj. 4 = 7-5 of the ocular micrometer 

 scale, of which each unit in terms of the known stage micrometer is (3-9/-/. 



7-5 x 6- 9 = 51- 75/>-- = • 05mm. 



(.05-^-25= -002 inch= 1/500 inch). 



•05 x 150=7-5 mm = the size of the figure. 



Figs. 2-8. Segmentation and formation of the embryo. 



Figs. 9-21. The larva. 



Fig. 22. The youngest spat. 



Fig. 2. Extrusion of the first polar body and lengthening of the 

 oosperm. 



Fig. 3. First (partial) cleavage into two blastomeres (micromeres) 

 above, with the uncleft deutomere below. 



Fig. 4. Second cleavage at right angles to the first, giving rise to four 

 micromeres. 



Fig. 5. Early stage of the morula. 



Fig. 6. Later stage of morula or beginning of the blastula, with the 

 deutomere still observable. 



Fig. 7. Early gastrula. 



Fig. 8. Later gastrula. 



Fig. 9. Trochophore or first stage of the larva, with its swimming 

 organ — the prototroch. The animal now begins to swim, swallow food, 

 and grow. 



Fig. 10. Beginning of the shell. 



Fig. 11. Growing shell. 



Fig. 12. Shell sufficiently large to enclose the body but not the 

 prototroch. Larvae may be raised to this stage from artificially fertilized 

 eggs. Larvae of this stage may also be caught by the plankton net in oy st er 

 waters early in July. Shell: length 10, depth 8, straight hinge-line 5. 



Fig. 13. Shell 15 x 14: 7. The prototroch is now a retractile velum. 



Fig. 14. Shell 20 x 18. Beginning of the umbos. 



Fig. 15. Shell 25 x 23. 



Fig. 16. Shell 30 x 28. Velum retracted within the shell. 



Fig. 17. Shell 35 x 32. 



