DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS 1207 



ination of the cells lining the celom (Fig. 1106). The orifice of the invagination 

 remains patent, and undergoes enlargement and modification to form the abdomi- 

 nal ostium of the uterine tube. The ducts pass backward lateral to the Wolffian 

 ducts, but toward the posterior end of the embryo they cross to the medial side 

 of these ducts, and thus come to lie side by side between and behind the latter 

 the four ducts forming what is termed the genital cord (Fig. 1109). The Miillerian 

 ducts end in an epithelial elevation, the Miillerian eminence, on the ventral part of 

 the cloaca between the orifices of the Wolffian 



ducts: at a later date they open into the cloaca Miillerian ducts 



in this situation. 



In the male the Miillerian ducts atrophy, 

 but traces of their anterior ends are repre- 

 sented by the appendices testis (hydatids of 

 Morgagni), while their terminal fused portions 

 form the utriculus in the floor of the prostatic 

 portion of the urethra (Fig. 1110, C). 



In the female the Miillerian ducts persist 

 and undergo further development. The por- 

 tions which lie in the genital core fuse to form 

 the uterus and vagina; the parts in front of 

 this cord remain separate, and each forms 

 the corresponding uterine tube the abdomi- 

 nal OStium of Which is developed from the FIG. 1109. Urogenital sinus of female human 



anterior extremity of the original tubular in- (fS^lfbyKeibe^" 

 vagination from the celom (Fig. 1110, B] . The 



fusion of the Miillerian ducts begins in the third month, and the septum formed 

 by their fused medial walls disappears from below upward, and thus the cavities 

 of the vagina and uterus are produced. About the fifth month an annular con- 

 striction marks the position of the neck of the uterus, and after the sixth month 

 the walls of the uterus begin to thicken. For a time the vagina is represented by a 

 solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at 

 the lower end of the uterus and marks the future vaginal fornices; about the fifth 

 or sixth month the lumen of the vagina is produced by the breaking down of the 

 central cells of the epithelium. The hymen represents the remains of the Miillerian 

 eminence. 



Genital Glands. The first appearance of the genital gland is essentially the 

 same in the two sexes, and consists in a thickening of the epithelial layer which 

 lines the peritoneal cavity on the medial side of the urogenital fold (Fig. 1106). 

 The thick plate of epithelium extends deeply, pushing before it the mesoderm and 

 forming a distinct projection. This is termed the genital ridge (Fig. 1106), and from 

 it the testis in the male and the ovary in the female are developed. At first the 

 mesonephros and genital ridge are suspended by a common mesentery, but as the 

 embryo grows the genital ridge gradually becomes pinched off from the meso- 

 nephros, with which it is at first continuous, though it still remains connected to 

 the remnant of this body by a fold of peritoneum, the mesorchium or mesovarium 

 (Fig. 1111). About the seventh week the distinction of sex in the genital ridge 

 begins to be perceptible. 



The Ovary. The ovary, thus formed from the genital ridge, is at first a mass 

 of cells derived from the celomic epithelium; later the mass is differentiated 

 into a central part or medulla (Fig. 1112) covered by a surface layer, the germinal 

 epithelium. Between the cells of the germinal epithelium a number of larger 

 cells, the primitive ova, are found, and these are carried into the subjacent stroma 

 by bud-like ingrowths (genital cords) of the germinal epithelium (Fig. 1113). 

 The surface epithelium ultimately forms the permanent epithelial covering of this 



