54 THE OYSTER. 



parent point makes its appearance on the surface of the 

 egg, increases in size, and soon forms a little, project- 

 ing, transparent knob the pole cell. 



Soon after, a second pole cell is formed, and the 

 female pronucleus unites with the male pronucleus to 

 form the nucleus of the developing egg, which egg now 

 becomes pear-shaped, with the pole cells at the broad 

 end of the pear, and this end soon divides into two 

 parts, so that the egg is now made of one large mass 

 and two slightly smaller ones, with the pole cells be- 

 tween them. Plate IV, Fig. 2. 



The later history of the egg shows that at this early 

 stage it is not perfectly homogeneous, but that the 

 protoplasm which is to give rise to certain organs of 

 the body has separated from that which is to give 

 rise to others. 



If the egg were split vertically 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 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 spheres 

 at the large end 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 soon divides up into smaller and smaller spherules, 

 Plate IV, Figs. 3, 4, 5, 6, until we have a layer of small 

 cells wrapped around the greater part of the surface of 

 a single large spherule. This spherule now divides 



