222 PROFESSOR HUXLEY ON THE ANATOMY 
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 
nexion 
obtains between the cavity of the ovisac and that of the duct, or not, I 
cannot certainly say. A marked constriction is generally 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 rh^d of an incn in diameter, 
that in fisr. 4, -rfnth. In each case the vitellus is somewhat more opake than in the pre- 
6- - c » 210 
vious 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 appearance is well 
shown in figs. 4 and 8 *, and it is worth recollecting 
fate of the yelk 
with the subsequent 
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 (tAo") and appearance. One or two 
minute clear spaces are to be seen in it, occasionally, in this and in other stages, but l 
suspect they are accidental. 
In the specimen represented in fig. 3, the duct appears to contain only a clear fluid 
as before, except that a few indistinct striae are visible towards its upper end. One 
would hardly know what to make of them, if it were not for the circumstance, that 
bundle of minute filaments, a few of which would readily give rise to the striation m 
question, hangs from the mouth of the duct. The filaments are sticking in its atria 
aperture by one end, while the remainder of their length protrudes. 
The filaments are exceedingly delicate, and so entangled that their individual dimen- 
sions cannot be estimated. The whole bundle, however, measures about 430th of an men 
length. The ends of the filaments inserted into the aperture are thickened, and more or 
less rod-like. In a slightly larger ovisac (fig. 4) no such filaments are visible about the 
mouth of the duct, but its upper dilated end contains a conical plug, composed of pre- 
cisely similar bodies, and a similar plug occupied a corresponding position in every 
ovisac, in this stasre. which I have examined. If the ovisac is not disposed in such a 
