40 Mr. J. A. Ryder on the Mieroscopic 
chemistry, that although both of these dyes, to begin with, 
are intimately mixed together in the staining-fluid, the diffe- 
rent histological elements of the section exert some. kind of 
selective power by which they absorb and hold mainly the 
one colour only. This peculiar property of the two colours, 
even when mixed together, enables one to distinctly map out 
the relations of the sexual elements in the reproductive fol- 
licles, the nuclei of the ovarian ova being stained red by the 
safranin, and the heads of the spermatozoa bluish green by 
the methyl green. The foregoing is mainly the method to 
which I have had recourse in working out the sexual charac- 
teristics of Ostrea edulis. Simpler staining-methods suffice 
in the case of Ostrea virginica and Ostrea angulata. A single 
colour used in staining sections of O. edulis is lable to lead 
to error, in consequence of the peculiar mode in which the 
spermatozoa are packed together in oblong clusters, which 
are often of about the size of the ovarian ova. This egg-like 
appearance of the masses of unripe spermatozoa in the fol- 
licles of the reproductive organs of the common oyster of 
Europe misled me when examining sections stained only 
with eosin or carmine. ‘The monochromatic effect produced 
by one colour only gave no hint as to the real relations of ova 
and spermatozoa in the follicles until high powers were used 
with special manipulation of the light. 
The characteristics of the reproductive organs of Ostrea 
edulis, O. virginica, and O. angulata are sufficiently marked 
to be very precisely described and figured, so as to enable any 
person to appreciate the differences, especially between the 
first and the last two. O. edulis is essentially hermaphroditic in 
the structure of its reproductive organs, while the other two 
are as distinctly moncecious or unisexual. A marked diffe- 
rence is also to be noted in the relative size or calibre of the 
reproductive follicles in the hermaphroditic and in the uni- 
sexual species. In O. edulis the calibre of the generative 
tubules appears to be relatively much greater than in O. vwir- 
ginica and O. angulata, nor are the tubules so densely 
crowded together as in the latter species. Up to this time this 
difference appears to me to be so marked that I think it would 
be possible to distinguish sections of O. edulis from those of 
the other two species by means of this one character. In 
other respects the history of the development of the repro- 
ductive tissues in both species appears to be similar. In 
all, the sexual tissue arises as a linear interstitial differentia- 
tion between the coarse connective-tissue cells of the animal, 
only that in O. edulis the rudimentary network does not form 
quite so close a meshwork as in the other two forms here 
