228 PROFESSOR HUXLEY ON THE ANATOMY 
disappears. On the other hand, on that side of the contents towards the wrinkled part 
of the membrane of the germinal vesicle, a number of minute, pale, spheroidal corpuscles 
make their appearance and spread over the face of the contents. Considering that, as 
we have seen, the germinal spot becomes pale before it ceases to be visible, and bearing 
in mind that the power of subdivision is one of the most characteristic properties of the 
class of bodies to which the germinal spot belongs, I do not think it very hazardous to 
assume that the corpuscles in question result from the division of the germinal spot. 
In all the ovisacs of this size the epithelium has undergone a very remarkable change. 
Instead of the thin cellular lamella which has previously lined the interior of the ovisac, 
a transparent substance excavated by many large spheroidal cavities of various sizes 
(which when the ovisac is viewed by a low power give it the appearance of being filled with 
numerous clear vesicles) occupies its cavity. 
Sixth Stage. Ovisacs about -j^th of an inch in diameter, in which the germinal vesicle 
has disappeared but a blastodermic membrane occupies its place. 
Figure 9 represents an ovisac of jfrst of an inch in diameter in situ. It will be observed 
that the duct is now very small in relation to the sac, and that the modified epithelium of 
the latter manifests the vesicular appearance characteristic of the later stages. The germ- 
inal vesicle is no longer to be seen, but, exactly in the position it ought to occupy, there is 
a patch of substance which, in profile (fig. 9), is obvious as a thick, darkish yellow line, but 
iowed from within or from without (fig. 9 a) is only visible under a high power, in con- 
sequence of the excessive paleness and delicacy of its components. It is, in fact, a very thin 
membrane y^th of an inch long and about half as wide, composed of a single layer of 
pheroidal, or more or less polygonal, corpuscles, each of which has an average diameter 
°f Wooth of an inch, though some are smaller and some are larger. Every one of these 
v 
its centre a small, apparently vesicular, more strongly refracting and 
hence more conspicuous body, usually not more than oAnth of an inch in diameter, but 
sometimes attaining to fully twice this diameter. That margin of this membrane which 
turned towards the upper aperture of the duct (fig. 9 a) is tolerably sharply defined, and 
has an evenly curved contour, so that this extremity of the patch has almost a semicir- 
cular outline. The rest of the membrane, on the other hand, has an elongated, irregular 
form, and less distinctly defined edges. 
In another ovisac ^th of an inch in diameter, there is the same entire absence of the 
germinal vesicle and the same presence of a delicate membrane of precisely the same 
ch aracters, but not more than T ^ ^nd of an inch in long diameter and ^ th of an inch 
wide. In this specimen the edge of the membrane which is turned towards the duct is 
still more distinctly semicircular, and it is almost as well-defined as the edge of the ger- 
minal vesicle in its latest condition, though no distinct membrane is discernible. The 
irregular part of the membranous disk bears a smaller proportion to the semicircular 
part, than in the preceding case. 
In each instance the membranous disk, which has been described, lies between the 
modified epithelium and the membrana propria. In position, therefore, it exactly corre- 
sponds with the germinal vesicle ; its colour, when the light passes through a thickness 
