46 Mr. J. A. Ryder on the Microscopic 
advanced in development in the same follicle, what is there 
to prevent self-fertilization ; in fact, what else can be the mode 
of reproduction ? 
In some of the sections of O. edulis examined by me, the 
ovules already measured 34, of an inch in diameter, showing 
them to be about twice the size of the ripe eggs of O. virginica 
and O. angulata, in both of which the ova are of about the 
same size when mature. Estimates which I have made, based 
on the figures of the eggs of O. edulis given by M. Davaine, 
show them to bez) of aninch in diameter. Estimates based 
on the figures of Lacaze-Duthiers give 3}; of an inch, while 
Mdbius and Horst give the size of the young fry at 74, of an 
inch in diameter. ‘The spherical heads of the spermatozoa of 
the three species here discussed measure about the same, or 
approximately y455 of an inch in diameter. The clusters of 
spermatozoa of O. edulis measure approximately 755 of an 
inch in diameter. ‘he spherical unsegmented spermatoblasts 
which break up into spermatozoa in QO. edulis measure jsp Of 
an inch in diameter. The nucleus of the ovarian eggs of O. 
edulis measures not quite 45 of an inch in diameter. The 
nucleus of the ovarian egg of O. anguluta measures approxi- 
mately y';; of an inch in diameter, which is about that 
of the nucleus of the egg of O. virginica. The large 
spherule of the nucleolus ot the egg of O. edulis measures 
sisy of an inch in diameter; the small spherale, which is 
stained red by the safranin, measures z3';5 of an inch; the 
long diameter of the conjoined spherules is 7757 of an inch. 
The long diameter of the nucleolus of the egg of O. angulata 
and QO. virginica is about zol5y Of an inch. A slide in my 
possession containing some of the brood of O. edulis shows 
that, even after it has acquired both vaives of the shell within 
the beard of the mother oyster, the brood varies greatly in 
size. I find, for example, that such fry measures from >} > of 
an inch down to as smallas ;4;5. This brood, like that of the 
American oyster, has not yet acquired any umbonal promi- 
nences at the hinge end of the valves. Before this occurs in 
the American-oyster embryo considerable growth has taken 
place ; but when the shell already covers the body the whoie 
embryo, contrary to what is found in the European species, 
measures little, if any, more in diameter than the egg, or 
about s4> of an inch. Later, when the embryo has grown 
considerably, and when it is on the eve of attaching itself 
permanently, it measures from 345 down to yo of an inch in 
diameter. The mode of fixation of the fry of both species is 
probably the same ; but the mode of incubation (the one in the 
mother, the other in the open water), we see, is widely diffe- 
