56 THE OVUM AND 



within the yelk in the first instance, and in each of these again 

 two new 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 to it. 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 by a 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 tying 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 



