ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 351 
Third Stage. Ovisacs under yhgth of an inch in diameter and in process 
of impregnation. (Pl. XXX. [Plate 29] figs. 3, 4, and 8*.) 
Of the two specimens in this stage which I have figured, the larger 
(fig. 3) is rather the less advanced. Its duct is longer than the 
diameter of the ovisac, and is not only hollow throughout its whole 
length, but, at its anterior end, opens into the atrium, with which, 
therefore, its cavity is in free communication. Whether a similar 
connexion obtains between the cavity of the ovisac and that of the 
duct, or not, I cannot certainly say. A marked constriction is gene- 
rally observable at the point of junction between the duct and the 
ovisac, corresponding to an inwardly projecting lip, which greatly 
narrows the apparent aperture of communication (fig. 4); and in 
some cases, the cellular wall of the lip appeared to have grown out, 
in such a manner as still further to diminish that aperture; but I 
have been unable, in any one instance, fully to assure myself of the 
closure of the passage. If, however, as I have reason to believe, the 
vitellus, in the fresh state, completely fills the ovisac, the aperture 
will be effectually closed by its means. 
The ovum in the ovisac represented in fig. 3, measures ;45nd of 
an inch in diameter, that in fig. 4,4,5th. In each case the vitellus is 
somewhat more opaque than in the previous stages ; but, as before, I 
have been unable to find any vitelline membrane, even when, as in 
fig. 8*, the ovum has been turned out of the ovisac. But I have 
constantly observed that while the greater part of the circumference 
of the yelk exhibits a well-defined dark contour, that portion which 
is away from the side on which the germinal vesicle lies, has a 
faint, hazy. outline, as if it were undergoing solution. This appear- 
ance is well shown in figs. 4 and 8%, and it is worth recollecting in 
connexion with the subsequent fate of the yelk. 
In both the ova represented in figs. 3 and 4, the germinal vesicle 
measures ;},th of an inch in diameter, and its contents are, as 
before, perfectly clear. A change of figure has accompanied its 
increase in size, for it is now oval, its long diameter being more or 
less perpendicular to the direction of the duct. Furthermore, it 
is situated at the surface of the ovum, at a point close to, but on one 
side of, the aperture of the duct; and that face which is nearest the 
surface of the vitellus is not unfrequently flattened. 
The germinal spot retains its previous size (¢s4;5”) and appearance. 
One or two minute clear spaces are to be seen in it, occasionally, in 
this and in other stages, but I suspect they are accidental. 
In the specimen represented in fig. 3, the duct appears to contain 
