58 THE OYSTER. 



PLATE IV. 



THE DEVELOPMENT OF THE OYSTER. 



All the figures are highly magnified, and all except 

 Figure 12 are reproduced from the author's drawings 

 from nature. Figure 12 is copied from a figure by 

 R. T. Jackson in the American Naturalist, December, 

 1890. 



FIGURE i. A newly laid egg with the male cells fas- 

 ened to its surface. 



FIGURE 2. An embryo in the two-celled stage. 



FIGURE 3. An embryo at the beginning of the four- 

 celled stage. 



FIGURE 4. An embryo at the end of the four-celled 

 stage. 



FIGURE 5. An embryo in the eight-celled stage. 



FIGURE 6. An embryo at the beginning of the six- 

 teen-celled stage. 



FIGURE 7. An optical section of an embryo at the 

 beginning of the swimming stage. sg t shell gland ; st, 

 stomach. 



FIGURE 8. A swimming embryo a little older than 

 the one shown in Figure 7. 



FIGURE 9. The stage in which the first traces of the 

 shell make their appearance, s, shell ; st, stomach. 



FIGURE 10. The swimming oyster, about three days 

 old. s, the edge of the shell ; st, stomach ; i, intestine. 



FIGURE 1 1 . A swimming oyster about six days old. 



FIGURE 12. An oyster which has become attached. 



