56 THE OVUM AND 
within the yelk in the first instance, and in each of these again 
two mew ones, and so on. 
We next proceed to consider the changes undergone by the 
external layer of cells furnished with nuclei. In eggs which 
have a diameter of a line, this entire membrane, if it may be so 
called, appears to be made up merely of cells. In such as have 
reached a higher stage of development, such as have a diameter of 
upwards of half an inch, for instance, it consists of two strata, 
the external of which is granulous, and no longer exhibits cells ; 
the internal, however, is composed of cells, which are flat, 
hexagonal, but also granulous, and bear the relation of a cover- 
ing of epithelium to the outer one. The external stratum 
passes away over the germinal vesicle and the foundation of the 
germinal membrane, so that these structures may easily be re- 
moved from its inner surface without injury toit. The internal 
cellular stratum, on the contrary, is interrupted at the spot where 
the germinal vesicle lies. I have not traced the mode of formation 
of this external granulous stratum through all its details ; I sup- 
pose it to be produced bya blending of the outer cells, which com- 
posed the original membrane when it was made up entirely of 
cells. As the period approaches at which the egg leaves the ovary, 
the epithelium-like stratum of cells gradually disappears, and the 
granulous membrane alone remains. It does not exhibit any 
disposition to unite with the structureless external membrane 
of the egg, even in eggs which are almost sufficiently mature for 
extrusion. If such an egg be cut open under water, and the 
investment derived from the ovary be drawn off, this granulous 
membrane frequently remains lying upon the yelk, whilst the 
structureless membrane follows the above-mentioned investment, 
and may readily be proved to be connected with it, when they 
are folded so that the inner surface forms a sharp edge. By 
the aid of the compressorium this structureless membrane may 
then be seen, projecting out from the border of the preparation. 
It often separates in large pieces during this manipulation, so 
that it has likewise no connexion with the parts pertaining to 
the ovary. If the signification of vitelline membrane is to be 
assigned to this structure, a blending between it and the granu- 
lous stratum must take place in the oviduct, in order to form 
the subsequent vitelline membrane of the extruded egg. 
We now pass on to that portion of the egg from which the 
