20 
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 
(Figure 6), 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 sep- 
arated from that which is to give rise to others. 
If the egg at the stage shown in Figure 6, were split in the 
plane of the paper, we should have what is to become one 
half of the body in one part an§ 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 
spherules at the small 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 in this 
and succeeding figures is to become the ventral surface of the 
adult oyster, and the surface which is on the right side in 
Figure 6 is to become the anterior end of the body of the 
adult. The figure therefore shows the half of the egg which 
is to become the left half of the body. The upper portion of 
the egg soon divides up into smaller and smaller spherules, 
until at the stage shown in Figures 24, 25 and 26, we have 
a layer of small cells wrapped around the greater part of 
the surface of a single large spherule, and the series of figures 
shows that the latter is the spherule which is below in Figure' 
6. This spherule now divides up into a layer of cells, and 
at the same time the egg, or rather the embryo, becomes 
flattened from above downward, and assumes the shape of 
a flat oval disk. Figures 29 and 30, are views of the up- 
per and lower surface of the embryo at about this time*. 
