;66 EMBRYOLOGY 



section of the tube. The mucous membrane is covered with cylindrical 

 ciliated epithelium, the movement of the cilia being from the ovary 

 toward the uterus. The membrane of the tubes has no mucous glands 

 (see Plate X, Fig. 5). 



It is not necessary to give a minute description of the external 

 organs of the female. Opening by the vulva externally, and termina- 

 ting at the neck of the uterus, is a membranous tube, the vagina. This 

 lies between the bladder and the rectum. It has a curved direction, 

 being 3^ to 3^ inches (8 to 9 centimeters) long in front, and 3^ to 4 

 inches (9 to 10 centimeters) long posteriorly. At the constricted por- 

 tion 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 presents a mucous membrane, 

 marked by transverse rugae, with papillae and mucous glands. Its sur- 

 face is covered with stratified epithelium in several layers. The vagina 

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

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

 longitudinal and circular muscular fibres of the non-striated variety. 

 Surrounding it, is a rather loose so-called 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, there is a true erectile 

 tissue. 



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

 bedded in the mass of granular cells which forms the discus proligerus. 

 Surrounding the ovum are cells similar to those found in other parts of 

 the membrana granulosa, and two or three layers of columnar cells, the 

 latter lying next the zona pellucida. These columnar cells constitute 

 the corona radiata. The ovum itself presents the following structures : 

 (i) Zona pellucida; (2) perivitelline space; (3) a clear outer zone of 

 the vitellus; (4) protoplasmic zone (formative yolk); (5) deutoplasmic 

 zone (nutritive yolk); (6) germinal vesicle (Purkinje); germinal spot 

 (Wagner). The thin membrane within the zona pellucida and immedi- 

 ately surrounding the vitellus, described under the name of vitelline 

 membrane by some anatomists, was not observed by Nagel in the 

 human ovum. 



The ovum is globular, with a diameter of about T | T of an inch (165 

 to 170 p) measured from the outer border of the zona pellucida. 



The zona pellucida (zona radiata, or vitelline membrane) is -^V o to 

 -j-^o of an inch (20 to 24 IJL) in thickness. It is a strong membrane, ap- 

 pearing in the form of a clear zone in the mass of surrounding cells. 



