STRUCTURE OF THE OVUM. 869 



& curved direction, being about four inches long in front, and five or six inches long pos- 

 teriorly. There is a constricted portion at the outer opening, where we have a muscle, 

 called the sphincter vaginas, and the tube is somewhat narrowed at its upper end, where 

 it embraces the cervix uteri. The inner surface presents a mucous membrane, marked by 

 transverse rugae, 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 



Fm. 279. External erectile organs of the female. (Lie<*eois.) 



A, pubis; B, B, ischinm ; C, clitoris ; O, 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. 



the passage of the child. It presents a proper coat of dense fibrous tissue, with longi- 

 tudinal and circular muscular fibres of the non-striated variety. We have, also, sur- 

 rounding it, a rather loose erectile tissue, which is most prominent at its lower portion. 



The parts composing the external organs are abundantly supplied with vessels and 

 nerves. In the clitoris, which corresponds to the penis of the male, and on either side 

 of the vestibule, we find a true erectile tissue. 



Structure of the Ovum. 



The ripe ovum lies in the Graafian follicle, embedded in the mass of cells which con- 

 stitutes the discus proligerus. Within the discus, surrounding the ovum, there seem to 

 be two kinds of cells ; first, cells evidently belonging to the Graafian follicle and similar 

 to the cells in other parts of the membrana granulosa ; second, 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, we can see, in the ovum, a clear, transparent membrane, a granu- 

 lar mass (the vitellus) filling this membrane completely, a large, clear nucleus, called the 

 germinal vesicle, and a nucleolus, called the germinal spot. 



The size of the ripe ovum, in the human subject and in mammals, is about T J y of an 

 inch, and its form is globular. 



The external membrane of the ovum is clear, apparently structureless, quite strong 

 and resisting, and it measures about FT Vff f an mcn m thickness. As it forms a trans- 

 parent ring in the mass of cells in which the ovum is embedded, this is sometimes called 

 the zona pellucida. According to recent researches, it seems that the primordial ovum 

 has at first no special investing membrane ; as it develops, it presents, surrounding the 



