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 -^oih of an inch in diameter, in lohlch the germinal vesicle 

 has disappeared but a blastodermic membrane occupies its place. 



Figure 9 represents an ovisac of -^rst of an inch in diameter in situ. It will be obsei'ved 

 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 yeUow line, but 

 viewed 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 jGotli of an incli long and about half as wide, composed of a single layer of 

 spheroidal, or more or less polygonal, corpuscles, each of which has an average diameter 

 of iVoo^h of an inch, though some are smaller and some are larger. Every one of these 

 contains in or near its centre a small, apparently vesicular, more strongly refracting and 

 hence more conspicuous body, usually not more than g-g-ooth of an inch in diameter, but 

 sometimes attaining to fully twice this diameter. That margin of this membrane which is 

 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- 

 ciilar outline. The rest of the membrane, on the other hand, has an elongated, irregular 

 form, and less distinctly defined edges. 



In another ovisac -gVth 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 

 characters, but not more than xg^nd of an inch in long diameter and 2~4oth 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- 

 f;j)onds with the germinal vesicle ; its colour, when tlie light passes through a thickness 



