Fig. 9. Eight-celled stage, side view, eighty-one minutes after fecundation. 

 Magnified 600 diameters. 



Fig. 10. Sixteen-celled stage, viewed from below, one hundred minutes after 

 fecundation. Note large yolk cell. Magnified 600 diameters. 



Fig. 11. Blastula stage, viewed from below, about nine hours after fecunda- 

 tion. The original egg has developed, by repeated divisions, into a mass of cells, 

 giving it a mulberry-like appearance. The large yolk cell has divided into four 

 macromeres, the rest of the cells constituting the micromeres. Magnified 600 

 diameters. 



Fig. 12. Ciliated gastrula, ten hours after fecundation. The embryo can now 

 swim through the water by means of the hair-like cilia. The larger cells have 

 become invaginated. Magnified 600 diameters. 



Fig. 13. Optical section of ciliated gastrula. Magnified 600 diameters. 



Fig. 14. Trochosphere stage, twelve to fourteen hours after fecundation. 

 The body has elongated and the cilia are now confined to the front end. Note 

 the long feeler or flagellum, which serves to guide the animal. The opening of the 

 primitive mouth can be seen on the lower side, while above is a slight indentation 

 corresponding to the beginning of the shell gland. Magnified 600 diameters. 



Fig. 15. Formation of the shell, which arises at two symmetrical points of 

 calcification, right and left of the median line, and gradually envelops the animal. 

 Magnified 600 diameters. 



Fig. 16. Early veliger larva, viewed from the side. The animal arrives at 

 this stage from seventeen to forty hours after fertilization, according to external 

 conditions. The duration of this stage is probably from five to six days, during 

 which the animal leads a free swimming life. Magnified 600 diameters. 



