320 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



tive vesicle, which does not disappear, but divides into 

 two parts, one of which is pushed out of the egg, and 

 becomes the polar globule, while the other remains behind 

 and becomes the nucleus of the developing egg, but 

 changes its appearance so that it is no longer conspicuous. 

 The egg now becomes pear-shaped, with the polar globule 

 at the broad end of the pear, and this end soon divides 

 into two parts, so that the egg (Fig. 161) is now made of 

 one large mass and two .slightly smaller ones, with the 

 polar globule between them. 



The later history of the egg shows that at this early 

 stage the egg is not perfectly homogeneous, but that the 

 protoplasm which is to give rise to 

 certain organs of the body has separ- 



/y i 1. 



^ ated from that which is to give rise 

 to others. 



FIG. 162. The same egg, ten minutes later, 

 in the same position. 

 FIG. 162. Letters as in Fig. 161. 



If the egg in the stage shown in Fig. 161, were split in 

 the plane of the paper, we should have what is to become 

 one half of the body in one part and the other half in the 

 other. The single spherule at the small end of the pear, 

 the macromere (a), is to give rise to the cells of the digestive 

 tract of the adult, and to those organs which are to be 

 derived from it, while the two spherules at the small end, 

 the micromeres (b and c), are to form the cells of the outer 

 wall of the body and the organs which are derived from it, 

 such as the gills, the lips and the mantle, and they are also 

 to give rise to the shell. The upper portion of the egg in 

 this and succeeding figures is to become the ventral sur- 

 face of the adult oyster, and the surface which is on the 



