Sexual Characteristics of Oysters. 41 
considered. ‘The tubules have a more extensive anastomiosis 
with each other in the unisexual species than in the herma- 
phroditic. In all the forms fine vessels pass off from the 
dorsal and ventral somatic arteries, which tend to branch into 
vessels of a capillary fineness amongst the reproductive follicles. 
Thus the glandular portions of the reproductive organs are 
effectively nourished by supplies of blood passing from the 
great vessels given off by the heart. ‘These are the principal 
characteristic features of the reproductive follicles in the her- 
maphroditic and unisexual forms which are noticed upon com- 
paring the two together. The most important differences 
between the two forms are to be found, however, in the mode 
in which the generative elements are produced in each type, 
which we will now consider. 
In O. edulis the reproductive glands, when well developed, 
show in many cases a lining of large nearly mature ovules 
or ovarian eggs at intervals ; and insinuated between them 
large coarsely granular bodies may be observed, in which large 
irregular nuclear bodies are often imbedded. These nuclear 
bodies are further distinguished from those of the ovules by 
their oval or oblong and often irregular form, and by con- 
taining a dense mass of granules which absorb safranin in 
such quantity as to become opaque. ‘This granular chro- 
matin, as it would be designated by Flemming, is usually 
ageregated at the centre of the nuclear or cellular mass, which- 
ever it may be, and is furthermore apt to conform to a certain 
extent to the external outline of the body which contains it. 
From these bodies the rounded granular cells appear to arise 
which fall into the cavity of the tubule or follicle, there to 
undergo further segmentation, and finally break up into sper- 
matozoa with spherical heads and filiform tails or flagella. 
Even (in some cases) where no spermatozoa are as yet re- 
vealed by the methyl green, these rounded spermogens or 
spermatoblasts are to be seen free in the centre of the follicles. 
Usually, however, the spermatoblasts have been crowded 
towards the external end of the tubule, where they have 
undergone differentiation into spermatozoa. ‘The sperma- 
tozoa are often on this account so crowded together at the 
outlet of the tubules, passing even into the superficial ducts, 
that when acted on by the methyl green they are revealed as 
a dense almost opaque dark bluish-green mass. The ovules, 
on the other hand, which may be quite nearly mature, remain 
unstained, except their spherical clear nucleus and nucleolus, 
which ig double, as if formed of two conjoined spherules. If 
the safranin has been washed out of the nucleus, the one 
spherule ef the nucleolus only is apt to retain the colour. The 
