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[7] OYSTER CULTIVATION IN THE NETHERLANDS. 1007 
the ectoderm. The last-mentioned cells gradually grow all round the 
vegetative cell or sphere without however inclosing it entirely. After- 
wards these cells also begin to segment, and a layer of cylindrical cells 
is formed, which is slightly invaginated or pushed inward at one point 
and which forms the entoderm. 
About the same time a crescent-shaped groove is formed at the other 
pole of the egg by an invagination or pushing inwards of the entodermal 
cells (erroneously called the gastrula mouth by former investigators), 
from whieh the shell-gland is developed. Soon after this we see the 
formation of the primary intestinal channel by a farther invagination 
of the entodermal surface. Back of the opening of the mouth 2 few large 
cells make their appearance, which must probably be considered as the 
first mesodermal cells. The shell-gland has now assumed the shape of 
a deep little bag, the wall of which is composed of large cylindrical cells. 
The portion of the ventral side located below the mouth projects con- 
siderably during the succeeding stage, so as to form a sort of foot, as in 
the embryo of the snail. 
Daring the further growth of the embryo the entoderm develops into 
@ Spacious stomach cavity, with a sac-like appendage, which later forms 
the end of the intestine, but which is still closed; the gastrula mouth 
becomes the permanent mouth. The ectoderm is now lifted up from 
close contact with the entoderm and in this way the body cavity is 
developed. The invagination of the shell-gland has meanwhile disap- 
peared almost entirely by an inflection of its margin. Its place is now 
occupied by a thickening of the ectoderm, consisting of long conical cells. 
These cells develop a thin membrane, which is the first trace of the 
shell, from which it appears that the bivalvular shell does not originally 
appear as such, but that the shell begins to form as one piece. Above 
the mouth a cirelet of vibratory filaments begins to make its appear- 
ance, which later forms the relum, the organ of motion of the larva. 
During the succeeding stage the shell rapidly increases in size, and 
almost entirely envelops the larva; the velum becomes more clearly de- 
fined, and in the center of the velar area, encircled by its ciliary ring or 
crown, a thickening of the ectoderm (velar plate) begins to make its ap- 
pearance. A funnel-shaped gullet forms the entrance to the pear-shaped 
stomach, whilst the hinder portion of the intestine develops an opening 
communicating with the outside. 
By the appearance of pigment at different points of the body of the 
larva (velar plale, gullet, stomach) the whktte spat assumes a dark gray 
color and becomes black spat. On both sides of the intestine muscles 
begin to develop, running downward from the hinge, by means of which 
the head part, which protrudes from the shell, can be drawnin; and like 
the abductor muscle, which connects the left and the right valves of the 
shell and closes them, the former are also developed from mesodermal 
- easier understood, if given in the original, than if a translation had been attempted: 
“‘entoderm,” ““mesoderm.” The similar term “‘¢cfoderm” is found in Webster, but not 
in Dunglison.—TRANSLATOR. 
