41 
(Die Austern rind die Austern-wirtschaft, p. 17) that the oys- 
ters discharge their ripe eggs into the gills, and that they 
'commence their development after they have left the repro- 
ductive organs 
The occurrence of unfertilized eggs on the gills is conclusive 
-evidence that impregnation does not always take place in the 
•ovaries, but it cannot be regarded as evidence that the eggs 
have not been discharged from the cloaca into the external 
water, and then drawn back into the mantle chamber. It 
seems possible that some, at least, of the eggs of the European 
oyster may be fertilized outside the shell, as is the case with 
the American species ; but there does not at present seem to 
be any reason to believe that they ever complete their devel- 
opment elsewhere than inside the shell. It is, of course, 
impossible for an American to decide this point, but I think 
it is one to which the renewed attention of European natural- 
ists might well be directed. 
SEGMENTATION. 
The segmentation of the oyster egg is remarkable for its 
;great rapidity, for its bilateral symmetry, and for the very 
well marked alternation of periods of activity with periods of 
rest. The rate of segmentation varies* greatly according to 
temperature and other conditions, but in some cases the cili- 
ated embryo is formed within two hours after fertilization. 
After the completion of the first division the position of the 
right and left sides, as well as that of the dorsal and ventral 
surfaces, is determined. 
The ripe, unfertilized egg is quite variable and irregular in 
shape, usually elongated and pear-shaped, but sometimes 
polyhedral, and without the stalk, round over part of its sur- 
face and flattened over part, or even perfectly spherical. The 
characteristic shapes of the eggs are well shown in 49, 51 
and 53. hi" o external membrane is visible in the unfertilized 
egg, and, as shown in the figures, the protoplasm of the 
yolk forms a thin, slightly granular layer around the very 
large oval, transparent germinative vesicle, which again con- 
tains a single, more highly refractive, germinative dot. In 
I 
