Very soon the
beginnings of the shell appear along the right and left sides of the
back of the embryo, and not long afterward a ciliated pad, the velum, is
formed along the under side. This velum can be thrust out from between
the valves of the shell at the will of the young animal, and used by the
motion of its cilia as an organ for driving food to the mouth, or
in swimming as a rudder. During these transformations the original
cream-white color of the germ changes into pale gray, and finally into a
deep bluish-gray color. At this time they have a long oval outline, and
are from 0.15 to 0.18 of a millimeter in breadth. Over 300,000 can find
room upon a square centimeter of surface. If an oyster in which the
embryos are in this condition is opened, there will be found upon its
beard a slimy coating thickly loaded with grayish-blue granules. These
granules are the embryo oysters, if a drop of the granular slime be
placed in a dish with pure sea water, the young animals will soon
separate from the mass, and spread swimming through the entire water.
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