ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 359 
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 character- 
istic 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 oth 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 ,»;st of an inch in diameter 7x 
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 
germinal 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 viewed from 
within or from without (fig. ga) is only visible under a high power, in 
consequence of the excessive paleness and delicacy of its components. 
It is, in fact, a very thin membrane ;},th of an inch 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 
zdyath 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 s¢ysth of an inch in diameter, but sometimes 
attaining to fully twice this diameter. That margin of this mem- 
brane which is turned towards the upper aperture of the duct (fig. 9a) 
is tolerably sharply defined, and has an evenly curved contour, so 
that this extremity of the patch has almost a semicircular outline. 
The rest of the membrane, on the other hand, has an elongated, 
irregular form, and less distinctly defined edges. 
