THE OYSTER. 49 



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 ^\n^ 

 rise to others. 



If the Q^<g 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 

 6gg soon divides up into smaller and smaller spherules, 

 Plate IV, Figs. 3, 4, 5, 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 

 up into a layer of cells, and at the same time the ^g,^, 

 or rather the embryo, becomes flattened from above 

 downward, and assumes the shape of a flat, oval disk. 

 In a sectional view it is seen to be made up of two 

 layers of cells ; an upper layer of small transparent 

 cells, which are to form the outer wall of the body, 

 and which have been formed by the division of the 

 spherules which occupy the upper end of the ^'g%y and 

 a lower layer of much larger, more opaque cells, which 

 are to become the walls of the stomach, and which have 

 been formed by the division of the large spherule. 



This layer is seen in the section to be pushed in a 

 little toward the upper layer, so that the lower surface 

 3 



