990 FEMALE ORGANS OF GENERATION. 



The developed vesicle lias two coats, viz., an external vascular tunic, 

 and an internal tunic named the ovicapsule, which is lined with a cellular or 

 epithelial layer, the membrana granulosa. At first the ovum appears near 

 the centre of the vesicle, while the latter is still very small, but, in the 

 mature condition, it lies towards the internal surface of the ovi-capsule, im- 

 bedded in the discus proligerus, a small flattened heap of granular cells, 

 continuous with the membrana granulosa. 



The ovum itself, first discovered in mammals by Baer, is a spherical body, 

 very constant in size, being about -j-i^th of an inch in diameter ; it consists 

 of a thick, colourless, and transparent envelope (zona pellucida), which 

 surrounds the substance of the yelk. Within the yelk, which is made up 

 of granular matter, is situated a still smaller vesicular body, named the 

 germinal vesicle, which is about -y^th of an inch in diameter ; and in this 

 again is an opaque spot, having a diameter only of -j-g- 1 ^ ^th to -^/^th of 

 an inch, and named the germinal spot (macula germinativa). 



The ova make their appearance in the ovary at so early a period that even 

 at the time of birth it has been found too late to observe their mode of origin. 

 It has been ascertained that the ovum makes its appearance before the 

 ovisac, and that the germinal vesicle is the first part of the ovum to be 

 formed, the granular substance of the yelk being gradually deposited round 

 it. Around the ovum a circle of cells becomes visible s grows thicker, 

 and divides into two layers, the outer of which becomes the membrana 

 granulosa, while the inner adheres to the ovum, forming the discus proli- 

 gerus. The precise nature and mode of origin of .the inner tunic of the 

 Graafian vesicle is matter of dispute, and indeed Henle denies that there 

 is any homogeneous membrane distinct from the outer cells of the membrana 

 granulosa. 



According to Schron's observations on the cat, the ova make their first appearance 

 near the surface of the ovary, and the vesicles become deeper placed tis they grow 

 larger : it is only in the later stages of growth, when the great expansion of the 

 vesicles presses aside the surrounding tissues, that they are again brought into con- 

 tact with the surface. From observations by Grohe it appears that the process is 

 similar in the human ovary. A beaded arrangement of the ova, as if developed in 

 anastomosing primitive gland tubes, has been observed by Valentin in the ovaries 

 of young animals, and more recently by Pfliiger in the adult cat. Spiegelberg finds 

 similar appearances in the human foetus. But the existence of such tubular glandular 

 structure and its relation to the commencing ova is still under discussion, and requires 

 farther observation for its determination. 



Fig. 689. OVUM OP THE Sow REMOVED FROM THE 

 GRAAFIAN VESICLE, WITH ITS CELLULAR COVERING (from 

 M. Barry). ^p 



1, germinal spot or nucleus ; 2, germinal vesicle ; 3, the 

 yelk ; 4, the zona pellucida or external covering of the 

 ovum ; 5, part of the tunica granulosa or proligerous disc ; 

 6, some adherent granules or cells. 



The Graafian vesicle, as it becomes more fully 

 5 4 , s dilated, approaches the surface of the ovary. By 



rupture of the vesicle the ovu-n, covered by the 

 cells of its proligerous disc, escapes into the Fallopian 

 tube, and is thus conveyed into the womb, while 



the ruptured vesicle becomes converted, by hypertrophy of its walls, into a 

 yellow mass named corpus luteum, which after persisting for a time, dwindles 

 down into a small fibrous cicatrix. 



