THE REPRODUCTIVE ORGANS. 63 T 



The human ovum measures about T J of an inch. Its external invest- 

 ment, or the zona pellucida, or vitclline membrane, is a transparent 

 membrane, about ^Vfr ^ an i ncn ^ n thickness, which under the micro- 

 scope appears as a bright ring (4, Fig. 427), bounded externally and in- 

 ternally by a dark outline. Within this transparent investment or zona 

 pellucida, and usually in close contact with it, lies the yolk or vitelhis, 

 which is composed of granules and globules of various sizes, imbedded 

 in a more or less fluid substance. The smaller granules, which are the 

 more-numerous, resemble in their appearance, as well as their constant 

 motion, pigment-granules. The larger granules or globules, which have 

 the aspect of fat-globules, are in greatest number at the periphery of the 

 yolk. The number of the granules is greatest in the ova of carnivorous 

 animals. In the human ovum their quantity is comparatively small. 



In the substance of the yolk is imbedded the germinal reside, or 

 yesicula germinativa (2, Fig. 427). The vesicle is of greatest relative 

 size in the smallest ova, and is in them surrounded closely by the yolk, 

 nearly in the centre of which it lies. During the development of the 

 ovum, the germinal vesicle increases in size much less rapidly than the 

 yolk, and comes to be placed near to its surface. It consists of a fine, 

 transparent, structureless membrane, containing a clear, watery fluid, in 

 which are sometimes a few granules; and at that part of the periphery of 

 the germinal vesicle which is nearest to the periphery of the yolk is situ- 

 ated the germinal spot, or macula germinativa, of a finely granulated ap- 

 pearance and of a yellowish color, strongly refracting the rays of light. 



Such are the parts of which the Graafian follicle and its contents, 

 including the ovum, are composed. With regard to the mode and order 

 of development of these parts there is considerable uncertainty. 



It appears that the Graafian follicles are formed in the following 

 manner: The embryonic ovary is covered with short columnar cells, or 

 the so-called germinal epithelium. The cells of this layer undergo pro- 

 liferation, so as to form several strata. These cells grow into the ovarian 

 stroma as longer or shorter columns or tubes. By degrees these tubes 

 become cut off from the surface epithelium, and form cell nests, small if 

 near the surface, larger if in the depth of the stroma. The nests in- 

 crease in size from multiplication of their cells, and may even give off 

 new nests laterally by constriction of them in various directions. Certain 

 of the cells of the germinal epithelium enlarge, and form ova; and the 

 formation of ova also takes place in the nests within the stroma. The 

 ova of a nest may multiply by division. The small cells of a nest sin-- 

 round the ova, and form their membrana granulosa, and the stroma 

 growing up separates the surrounded ova into so many Graafian follicles. 

 The other layers, namely, the membrana fibrosa and the membrana 

 vasculosa, are derived from the stroma. 



The smallest follicles are formed at the surface, and form the cortical 



