STRUCTURE OF THE OVUM. 



777 



centimetres) long in front, and five to six inches (12'7 to 15'2 centimetres) 

 long posteriorly. At the constricted portion of the outer opening, there is 

 a muscle, called the sphincter vaginae, and the tube is somewhat narrowed 

 at its upper end, where it embraces the cervix uteri. The inner surface pre- 

 sents a mucous membrane, marked by transverse rugas, with papillae and 

 mucous glands. Its surface is covered with flattened epithelium. The vagina 

 is quite extensible, as it must be during parturition, to allow the passage of 

 the child. It pre- 

 sents a proper coat 

 of dense, fibrous tis- 

 sue, with longitud- 

 inal and circular 

 muscular fibres of 

 the non-striated va- 

 riety. Surrounding 

 it, is a rather loose, 

 so-called erectile tis- 

 sue, which is most 

 prominent at its 

 lower portion. 



The parts com- 

 posing the external 

 organs are abund- 

 antly supplied with 

 vessels and nerves. 

 In the clitoris,which 

 corresponds to the 

 penis of the male, and on either side of the vestibule, there is a true, erectile 

 tissue. 



Structure of the Ovum. The ovum lies in the Graafian follicle, embedded 

 in the mass of cells which constitutes the discus proligerus (Von Baer, 1827). 

 Within the discus, surrounding the ovum, there seem to be two kinds of 

 cells ; (1) cells evidently belonging to the Graafian follicle and similar to the 

 cells in other parts of the membrana granulosa ; (2) a single layer of columnar 

 cells belonging to the ovum and probably concerned in the production of the 

 proper membrane of the ovum, the vitelline membrane. Regarding the 

 vitelline membrane as the external covering, the ovum appears to be com- 

 posed of (1) a clear, transparent membrane ; (2) a granular mass (the vitellus) 

 filling this membrane completely ; (3) a large, clear nucleus, called the ger- 

 minal vesicle ; and (4) a nucleolus, called the germinal spot. 



The diameter of the ripe ovum, in the human subject and in mammals, 

 is about T ^-g- of an inch (200 /*.), and its form is globular. The external mem- 

 brane of the ovum is clear, marked by fine, transverse lines, quite strong and 

 resisting, and measures about -^gVir f an i ncn (10 /A) in thickness. It appears 

 as a transparent ring in the mass of cells in which the ovum is embedded, 

 and forms what is called the zona pellucida. The primordial ovum has at 



Fro. 283. External erectile organs of the female (Liegeois). 

 A, pubis ; B, B, ischium ; C, clitoris ; D, gland of the clitoris ; E, bulb ; F, 

 constrictor muscle of the vulva ; G, left pillar of the clitoris ; H, dorsal 

 vein of the clitoris ; I, intermediary plexus ; J, vein of communication 

 with the obturator vein ; K, obturator vein ; M, labia minora. 



