KEY TO PLATE V 



Fig. 1. Oyster ova, under low power, magnification 50 diameters. 



Fig. 2. Oyster sperm, under low power. 



Fig. 3. Ovum, under higher power, magnified 250 diameters, m. 

 membrane; ec, ectoplasm; en, endoplasm; n, nucleus; nc, nucleolus. 



Fig. 4. Spermatozoa, a, magnified 250; be, magnified 1,000 times. 



Fig. 5. Ovum and spermatozoon about to meet prior to fertilization. 



Fig. 6. After entrance of the spermatozoon into the ovum, as, astro- 

 sphere; c, centrosome; r, rays of the spindle; c/i, chromosomes; sp, 

 head of spermatozoon. 



Fig. 7. Extrusion of first polar body, p v 



Fig. 8. Second polar body. p 2 . 



Fig. 9. $ pn, female pronucleus. 

 ^ pn, male pronucleus. 



Fig. 10. sgn, segmentation nucleus. 



Fig. 11. First cleavage. 



Fig. 12. Resting stage. 



Fig. 13. Second cleavage. 



Fig. 14. Third cleavage. 



Fig. 15. Fourth cleavage. 



Fig. 16. Diagram of cleavage from Korschelt and Heider, Part IV., 

 fig. 11. 



Fig. 17. Horst's fig. 3. 



Fig. 18. Brooks' fig. 14. 



Fig. 19. Median sagittal plane to show the process of invagination. 

 ec, ectoderm; en, endoderm; sc, segmentation cavity. 



Fig. 20. Horst's fig. 8 turned upside down to bring the blastopore, 

 o, below, and the preconchylian (shell) gland, sk, above. This is a later 

 gastrula than fig. 19. 



Fig. 21. Horst's fig. 13 rotated to the same position as the other 

 figures for ease of comparison, g, gastric cavity; ms, mesoderm; s, shell. 

 The gastrula shows also ectoderm, shell-gland, cilia of velum, endoderm, 

 blastopore, segmentation cavity. 



Fig. 22. Brooks' fig. 32. Surface view of embyro (larva) from right 

 side. 



Fig. 23. Brooks' fig. 33. Optical section of preceding (compare 

 with fig. 21). 



Fig. 24. Brooks' fig. 36. Beginning of shell-valve, down on near side. 



Fig. 25. Brooks' fig. 37. Beginning of shell-valve, up on near side. 



Fig. 26. Brooks' fig. 44. Bight side of embryo (larva) six days old. 



