In Mammalia the middle part of the duct, known as Gaertner’s canal, persists in the females of some monkeys, of the pig and of many ruminants.
The Wolffian body atrophies nearly completely in both sexes; though, as described above, part of it opposite the testis persists as the head of the epididymis. The posterior part of the gland from the level of the testis may be called the sexual part of the gland, the anterior part forming the non-sexual part. The latter, i.e. the anterior part, is first absorbed; and in some Reptilia the posterior part, extending from the region of the genital glands to the permanent kidney, persists till into the second year.
Various remnants of the Wolffian body are found in the adults of both sexes in different types. The most constant of them is perhaps the part in the female equivalent to the head of the epididymis and to parts also of the coiled tube of the epididymis, which may be called, with Waldeyer, the epoophoron[261]. This is found in Reptiles, Birds and Mammals; though in a very rudimentary form in the first-named group. Remnants of the anterior non-sexual part of the Wolffian bodies have been called by Waldeyer parepididymis in the male, and paroophoron in the female. Such remnants are not (Braun) found in Reptilia, but are stated to be found in both male and female Birds, as a small organ consisting of blindly ending tubes with yellow pigment. In some male Mammals (including Man) a parepididymis is found on the upper side of the testis. It is usually known as the organ of Giraldes.
The Müllerian duct forms, as has been stated, the oviduct in the female. The two ducts originally open independently into the cloaca, but in the Mammalia a subsequent modification of this arrangement occurs, which is dealt with in a separate section. In Birds the right oviduct atrophies, a vestige being sometimes left. In the male the Müllerian ducts atrophy more or less completely.
In most Reptiles and in Birds the atrophy of the Müllerian ducts is complete in the male, but in Lacerta and Anguis a rudiment of the anterior part has been detected by Leydig as a convoluted canal. In the Rabbit (Kölliker)[262] and probably other Mammals the whole of the ducts probably disappears, but in some Mammals, e.g. Man, the lower fused ends of the Müllerian ducts give rise to a pocket opening into the urethra, known as the uterus masculinus; and in other cases, e.g. the Beaver and the Ass, the rudiments are more considerable, and may be continued into horns homologous with the horns of the uterus (Weber).
The hydatid of Morgani in the male is supposed (Waldeyer) to represent the abdominal opening of the Fallopian tube in the female, and therefore to be a remnant of the Müllerian duct.
Changes in the lower parts of the urinogenital ducts in the Amniota.
The genital cord. In the Monodelphia the lower part of the Wolffian ducts becomes enveloped in both sexes in a special cord of tissue, known as the genital cord ([fig. 407], gc), within the lower part of which the Müllerian ducts are also enclosed. In the male the Müllerian ducts in this cord atrophy, except at their distal end where they unite to form the uterus masculinus. The Wolffian ducts, after becoming the vasa deferentia, remain for some time enclosed in the common cord, but afterwards separate from each other. The seminal vesicles are outgrowths of the vasa deferentia.
In the female the Wolffian ducts within the genital cord atrophy, though rudiments of them are for a long time visible or even permanently persistent. The lower parts of the Müllerian ducts unite to form the vagina and body of the uterus. The junction commences in the middle and extends forwards and backwards; the stage with a median junction being retained permanently in Marsupials.
The urinogenital sinus and external generative organs. In all the Amniota, there open at first into the common cloaca the alimentary canal dorsally, the allantois ventrally, and the Wolffian and Müllerian ducts and ureters laterally. In Reptilia and Aves the embryonic condition is retained. In both groups the allantois serves as an embryonic urinary bladder, but while it atrophies in Aves, its stalk dilates to form a permanent urinary bladder in Reptilia. In Mammalia the dorsal part of the cloaca with the alimentary tract becomes first of all partially constricted off from the ventral, which then forms a urinogenital sinus ([fig. 407], ug). In the course of development the urinogenital sinus becomes, in all Mammalia but the Ornithodelphia, completely separated from the intestinal cloaca, and the two parts obtain separate external openings. The ureters ([fig. 407], 3) open higher up than the other ducts into the stalk of the allantois which dilates to form the bladder (4). The stalk connecting the bladder with the ventral wall of the body constitutes the urachus, and loses its lumen before the close of embryonic life. The part of the stalk of the allantois below the openings of the ureters narrows to form the urethra, which opens together with the Wolffian and Müllerian ducts into the urinogenital cloaca.