21 
In a sectional view, Figures 1, it is seen to be made up of 
two layers of cells; an upper layer of small transparent cells, 
ec , which are to form the outer w^all of the body, and which 
have been formed by the division of the spherules which 
occupy the upper end of the egg in Figure 25, and a low r er 
layer of much larger, more opaque cells, y, which are to be- 
come the walls of the stomach, and which have been formed 
by the division of the large spherule, a, of Figure 25. 
This layer is seen in the section to be pushed in a little to- 
wards the upper layer, so that the lower surface of the disk- 
shaped embryo is not flat, but very slightly concave. This 
concavity is destined to grow deeper until its edges almost 
meet, and it is the rudimentary digestive cavity. A very 
short time after this stage has been reached, and usually with- 
in from two to four hours after the eggs were fertilized, the 
embryo undergoes a great change of shape, and assumes the 
form which is shown in three different views in Figures 32, 
33, 34 and 35. 
A circular tuft of long hairs or cilia has now made its ap- 
pearance at what is thus marked as the anterior end of the 
body, and as soon as these hairs are formed they begin to 
swing backwards and forwards in such a way as to constitute 
a swimming organ, which rows the little animal up from 
the bottom to the surface of the water, where it swins around 
very actively by the aid of its cilia. This stage of develop- 
ment, Figure 32, which is of short duration, is of great 
importance in raising the young oysters, for it is the time when 
they can best be siphoned off into a separate vessel and freed 
from the danger of being killed by the decay of any eggs which 
may fail to develop. On one surface of the body at this stage, 
the dorsal surface, there is a well marked groove, and when a 
specimen is found in a proper position for examination, the 
opening into the digestive tract is found at the bottom of 
this groove. Figure 33, is a sectional view of such an 
embryo. It is seen to consist of a central cavity, the diges- 
tive cavity, which opens externally on the dorsal surface of 
the body by a small orifice, the primitive mouth, and which 
is surrounded at all points, except at the mouth, by a wall 
