The development of the reproductive organs is so closely inter woven with that of the urinary that some reference from this article to that on the URINARY SYSTEM is necessary. It will here be convenient to take up the development at the stage depicted in the accompanying figure (fig. 3), in which the genital ridge is seen on each side of the attachment of the mesentery ; external to this, and forming another slight ridge of its own, is the Wolffian duct, while a little later the Miillerian duct is formed and lies ventral to the Wolffian. The early history of these ducts is indicated in the article on the URINARY SYSTEM. Until the fifth or sixth week the development of the genital ridge is very much the same in the two sexes, and consists of cords of cells growing from the epithelium-covered surface into the mesenchyme, which forms the interior of the ridge. In these cords are some large germ cells which are distinguishable at a very early stage of development. It must, of course, be understood that the germinal epithelium covering the ridge, and the mesenchyme inside it, are both derived from the mesoderm or middle layer of the embryo. About the fifth week of human embryonic life the tunica albu ginea appears in the male, from which septa grow to divide the testis into lobules, while the epithelial cords form the seminiferous tubes, though these do not gain a lumen until just before puberty. From the adjacent mesonephros, or perhaps, coelomic epithelium, cords of cells grow into the attached part of the genital ridge, or testis, as it now is, and from these the rete testis is developed.
In the female the same growth of epithelial cords into the mesenchyme of the genital ridge takes place, but each one is distinguished by a bulging toward its middle, in which alone the large germ cells are found. Eventually this bulging part is broken up into a series of small portions, each of which contains one germ cell or ovum and gives rise to a Graafian follicle. Mesonephric cords appear as in the male ; they do not enter the ovary, however, but form a transitory network (rete ovarii) in the mesovarium. As each genital gland enlarges it remains attached to the rest of the intermediate cell mass by a constricted fold of the coelomic membrane, known as the mesorchium in the male, and the mesovarium in the female. Lying dorsal to the genital ridge in the intermediate cell mass is the mesonephros, consisting of numerous tubules which open into the Wolffian duct. This at first is an important excretory organ, but during development becomes used for other purposes. In the male, as has been shown, it may form the rete testis, and certainly forms the vasa efferentia and globus major of the epididymis : in addition to these, some of its separate tubes probably account for the vas aberrans and the organ of Giraldes (see fig. 4). In the female the tubules of
the epoophoron represent the main part, while the paroophoron, like the organ of Giraldes in the male, is probably formed from some separate tubes (see fig. 4).
The Wolffian duct, which, in the early embryo, carries the excretion of the mesonephros to the cloaca, forms eventually the body and tail of the epididymis, the vas deferens, and ejaculatory duct in the male, the vesicula seminalis being developed as a pouch in its course. In the female this duct is largely done away with, but remains as the collecting tube of the epoi5phoron; and in some mammals as the duct of Gartner, which runs down the side of the vagina to open into the vestibule.
The Miillerian duct, as it approaches the cloaca, joins its fellow of the opposite side, so that there is only one opening into the ventral cloacal wall. In the male the lower part only of it remains as the uterus masculinus (fig. 4), but in the female the Fallopian tubes, uterus, and probably the vagina, are all formed from it (fig. 4). In both sexes a small hydatid or vesicle is liable to be formed at the beginning of both the Wolffian and Miillerian duct (fig. 4) ; in the male these are close together in front of the globus major of the epididymis, and are known as the sessile and pedunculated hydatids of Mor gagni. In the female there is a hydatid among the fimbriae of the Fallopian tube which of .course is Miillerian and corresponds to the sessile hydatid in the male, while another is often found at the beginning of the collecting tube of the epoophoron and is probably formed by a blocked mesonephric tubule. This is the pedunculated hydatid of the male. The development of the vagina, as Berry Hart (bourn. Anat. and Phys. xxxv. 33o) has pointed out, is peculiar. Instead of the two Miillerian ducts joining to form the lumen of its lower third, as they do in the case of the uterus and its upper two-thirds, they become obliterated, and their place is taken by two solid cords of cells, which later become canalized and the septum between them is obliterated.
The common chamber, or cloaca, into which the alimentary, urinary and reproductive tubes open in the foetus, has the urinary bladder (the remains of the allantois) opening from its ventral wall (see PLACENTA and URINARY SYSTEM).
During development the alimentary or anal part of the cloaca is separated from the urogenital. According to F. Wood Jones, the anal part is completely shut off from the urogenital and ends in a blind pouch which grows toward the surface and meets a new ectodermal depression, the permanent anus, not being part of the original cloacal aperture, but a new perforation. This de scription is in harmony with the malformations occurring in this region.