DEVELOPMENT OF URINARY AND GENERATIVE ORGANS 1425 



Wolffian body. 



The Mullerian Ducts. Shortly after the formation of the Wolffian ducts a second pair of 

 ducts is developed. These are named the Mullerian ducts. Each arises on the outer aspect of 

 the corresponding Wolffian body as a tubular invagination of the cells lining the ccelom (Fig. 

 1177). The orifice of the invagination remains patent, and undergoes enlargement and modi- 

 fication to form the abdominal ostium of the Fallopian tube. The ducts pass backward on the 

 outer aspects of the Wolffian bodies, but toward the posterior end of the embryo they cross to the 

 inner side of the Wolffian 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. 1180). 



Ultimately, the Mullerian ducts open into the ventral part of the cloaca between the orifices of 

 the W'olffian ducts, and terminate on an elevation named the Mullerian eminence (Fig. 1180). 



In the male the Mullerian ducts atrophy, but traces of their anterior ends are represented bv 

 the sessile hydatids of the epididymis, while their terminal fused portions form the uterus mascu- 

 linus or sinus pocularis in the floor of the prostatic portion of the urethra (Fig. 1183). 



In the female, the MiiUerian ducts persist and undergo further development. The portions 

 which lie in the genital cord fuse to form the uterus and vagina; the parts in front of this cord re- 

 main separate, and each forms the correspond- 

 ing Fallopian tube the abdominal ostium of 

 which is developed from the anterior extremity 

 of the original tubular invagination from the 

 ccelom (Fig. 1 182). The fusion of the Miiller- M iillerian *"* \ 



j.u xu- j xu j xu Wolffian duct i\ 



ian ducts begins in the third month, and the 

 septum formed by their fused mesal walls 

 disappears from below upward, and thus the 

 cavities of the vagina and uterus are produced. 

 About the fifth month an annular constriction 

 marks the position of the neck of the uterus, 

 and after the sixth month the walls of the uterus 

 begin to thicken. The development of the 

 vagina in the manner just described would 

 necessitate the growth of a septum between it 

 and the urethra; but Wood-Jones maintains 

 that no such septum exists, and that "the 

 vagina is, for a great part of fetal life, a solid 

 rod, and not an open canal at all." He says: 

 "Early in the history of the embryo the 

 Mullerian ducts open into the urogenital 

 sinus at its upper part; late in its history 

 they open at the hind-end of the vagina, and 

 for a considerable interval they have no open- 

 ing at all the old one being lost and the new eight and"a half to nine weeks old. ^From model by 

 one not yet formed. No septal division is Keibe1 -) 

 employed in this change; but as the hindgut, 



when its cloacal opening is lost, reestablishes communication with the exterior by a new 

 downgrowth, so the Mullerian ducts, when their cloacal opening becomes obliterated, tunnel 

 a new passage to the hind end." 



Genital Gland. 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 or body cavity 

 on the inner side of the Wolffian ridge. Beneath the thickened epithelium an increase in the 

 mesoderm takes place, forming a distinct projection. This is termed the genital ridge (Fig. 1118), 

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

 fian body and genital ridge are suspended by a common mesentery, but as the embryo grows the 

 genital ridge gradually becomes pinched off from the W'olffian body, with which it is at first con- 

 tinuous, though it still remains connected to the remnant of this body by a fold of peritoneum, 

 the mesorchium or mesovarium (Fig. 1184). About the seventh week the distinction of sex in 

 the genital ridge begins to be perceptible. 



The ovary, thus formed from the genital ridge, consists of a central part of connective tissue 

 covered by a layer of epithelium, 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 of the germinal epithelium, the cells of which surround the 

 primitive ova; in this manner the primitive Graafian follicles are formed. The rest of the ger- 

 minal epithelium on the surface of the ovary forms the permanent epithelial covering of this 

 organ (Fig. 1185). According to Beard, the primitive ova are early set apart during the segmen- 

 tation of the ovum and migrate into the germinal ridge. 



Waldeyer taught, and for many years his views have been accepted, that the primitive germ 

 cells are 'derived from the "germinal epithelium" covering the genital ridge. Beard, 1 on the 

 i Journal of Anatomy and Physiology, vol. xxxviii. 



90 



<-d -Spinal cord. 



Spinal ganglion. 



f~Notochord. 

 r~Sympathetic ganglion. 



Inferior vena cava. 



Common iliac artery. 



Ureter. 



Mesovarium. 



Intestine. 



Bladder. 



Umbilical artery. 



FIG. 1184. Transverse section of human embryo of 



