[13] THE OYSTER AND OYSTER-CULTURE. 695 
water. When the embryos are at this stage their number may be esti- 
mated in the following manner: The whole mass of embryos is carefully 
seraped from the beard of the mother oyster by means of a small hair- 
brush. The whole mass is then weighed, and afterwards a small portion 
of the mass. This small portion is then diluted with water or spirits of 
wine, and the embryos portioned out into a number of small glass dishes, 
FiaG. 7. 
A few stages of development of the embryo oyster; a to e enlarged 125 times, f and 
g, 150 times. 
a. The freshly-produced egg. In the yolk-mass is seen the germinative vesicle, 
with its nucleus. 
b. Commencement of development. A part of the vesicle has passed out. 
c. Division of the egg into two unequal portions. 
d. A later stage. 
e. The germ now consists of a layer of cells, which have arisen by repeated divis- 
ions of previous cells. They form a hollow vesicle, with a depression upon one side, 
which is the beginning of the digestive system. 
jf. The embryo is now represented at about the stage at which it leaves the brood- 
cavity. It has a transparent two-valved shell, and inside of the body the course of 
the digestive tract can be made out. An arrow shows the position of the mouth, and 
those within the body indicate the course which the food takes. Behind the wsoph- 
agus is the stomach, with two enlargements. The end of the intestine is shown over 
the mouth. To the left of the first enlargement of the stomach is the shell-muscle. 
On the under side is the velum, which is the locomotive organ of the young oyster. 
The young oyster can, by means of muscles, draw the velum entirely within the shell. 
g. An embryo, seen from behind. Upon the sides are seen the valves of the shell, 
and across the body, from one valve to another, passes the shell-muscle. Below this 
muscle is the velum, with the muscles, one on each side, which serve to withdraw it 
into the shell. 
so that they can be placed under the microscope and counted. Thus, 
knowing the weight of the small portion and the number of embryos in 
