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. ■\¥hether a similar con- 

 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 narroAvs 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 stUl 

 further to diminish that aperture ; but I have been unable, in any one instance, fully to 

 assiu'c 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 Tg^ii'l of an inch in diameter, 

 that in fig. 4, -jloth. In each case the vitellus is somewhat more opake than in the pre- 

 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 

 shoAvn 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 4ioth 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 (xiiVo") ^^^ 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 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 a 

 bundle of minute filaments, a few of which would readily give rise to the striation in 

 question, hangs from the mouth of the duct. The filaments are sticking in its atrial 

 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 4 3 o'th of an inch in 

 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 other 

 ovisac, in this stage, which I have examined. If the ovisac is not disposed in such a 



