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Rh while the paroöphoron, like the organ of Giraldès in the male, is probably formed from some separate tubes (see fig. 8, Ep. O. and Par. O.).

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 epoöphoron, and in some mammals as the duct of Gärtner, which runs down the side of the vagina to open into the vestibule.

The Müllerian 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. 8, U.M.), but in the female the Fallopian tubes, uterus, and probably the vagina, are all formed from it (fig. 8, F.T. and U.). In both sexes a small hydatid or vesicle is liable to be formed at the beginning of both the Wolffian and Müllerian duct (fig. 8, P.H. and S.H.); 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 Morgagni. In the female there is a hydatid among the fimbriae of the Fallopian tube which of course is Müllerian and corresponds to the sessile hydatid in the male, while another is often found at the beginning of the collecting tube of the epoöphoron 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 (Journ. Anat. and Phys. xxxv. 330) has pointed out, is peculiar. Instead of the two Müllerian 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 Hart thinks are derived from the Wolffian ducts and are therefore probably of ectodermal origin, though this is open to doubt. These cords 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 and ).

During development the alimentary or anal part of the cloaca is separated from the urogenital, and in the article the hitherto accepted method of this separation is described. The question has, however, lately been reinvestigated by F. Wood Jones, who says that 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 main point being that the permanent anus is not, according to him, any part of the original cloacal aperture, but a new perforation. This description is certainly more in harmony with the malformations occurring in this region than the old one, and only awaits confirmatory evidence to be generally accepted.

The external generative organs have at first the same appearance in the two sexes, and consist of a swelling, the genital eminence, in the ventral wall of the cloaca. This in the male becomes the penis and in the female the clitoris. Throughout the generative system the male organs depart most from the undifferentiated type, and in the case of the genital eminence two folds grow together and enclose the urogenital passage, thus making the urethra perforate the penis, while in the female these two folds remain separate as the labia minora or nymphae. Sometimes in the male the folds fail to unite completely, and then there is an opening into the urethra on the under surface of the penis—a condition known as hypospadias.

In the undifferentiated condition the integument surrounding the genital opening is raised into a horseshoe like swelling with its convexity over the pubic symphysis and its concavity toward the anus; the lateral parts of this remain separate in the female and form the labia majora, but in the male they unite to form the scrotum. The median part forms the mons Veneris or mons Jovis.

The Descent of the Testis.—It has been shown that the testis is formed in the loin region of the embryo close to the kidney, and it is only in the later months of foetal life that it changes this position for that of the scrotum. In the lower part of the genital ridge a fibro-muscular cord is formed which stretches from the lower part of the testis to the bottom of the scrotum; it is known as the gubernaculum testis, and by its means the testis is directed into the scrotum. Before the testis descends, a pouch of peritoneum called the processus vaginalis passes down in front of the gubernaculum through the opening in the abdominal wall, which afterwards becomes the inguinal canal, into the scrotum, and behind this the testis descends, carrying with it the mesonephros and mesonephric duct. These, as has already been pointed out, form the epididymis and vas deferens. At the sixth month the testis lies opposite the abdominal ring, and at the eighth reaches the bottom of the scrotum and invaginates the processus vaginalis from behind. Soon after birth the communication between that part of the processus vaginalis which now surrounds the testis and the general cavity of the peritoneum disappears, and the part which remains forms the tunica vaginalis. Sometimes the testis fails to pass beyond the inguinal canal, and the term “cryptorchism” is used for such cases.

In the female the ovary undergoes a descent like that of the testis, but it is less marked owing to the fact that the gubernaculum becomes attached to the Müllerian duct where that duct joins its fellow to form the uterus; hence the ovary does not descend lower than the level of the top of the uterus, and the part of the gubernaculum running between it and the uterus remains as the ligament of the ovary, while the part running from the uterus to the labium is the round ligament. In rare cases the ovary may be drawn into the labium just as the testis is drawn into the scrotum.

Comparative Anatomy.—In the Urochorda, the class to which Salpa, Pyrosoma and the sea squirts (Ascidians) belong, male and female generative glands (gonads) are present in the same individual; they are therefore hermaphrodite.

In the Acrania (Amphioxus) there are some twenty-six pairs of gonads arranged segmentally along the side of the pharynx and intestine and bulging into the atrium. Between them and the atrial wall, however, is a rudimentary remnant of the coelom, through which the spermatozoa or ova (for the sexes are distinct) burst into the atrial cavity. There are no genital ducts.

In the Cyclostomata (lampreys and hags) only one median gonad is found, and its contents (spermatozoa or ova) burst into the coelom and then pass through the genital pores into the urogenital sinus and so to the exterior. It is probable that the single gonad is accounted for by the fact that its fellow has been suppressed.

In the Elasmobranchs or cartilaginous fishes there are usually two testes or two ovaries, though in the dogfish one of the latter is suppressed. From each testis, which in fish is popularly known as the soft roe, vasa efferentia lead into the mesonephros, and the semen is conducted down the vas deferens or mesonephric duct into the urogenital sinus, into which also the ureters open. Sometimes one or more thin-walled diverticula—the sperm sacs—open close to the aperture of the vas deferens. In the female the ova are large, on account of the quantity of yolk, and they burst into the coelum, from which they pass into the large Müllerian ducts or oviducts. In the oviparous forms, such as the common doglish (Scyllium), there is an oviducal gland which secretes a horny case for the egg after it is fertilized, and these cases have various shapes in different species. Some of the Elasmobranchs, e.g., the spiny dogfish (Acanthias), are viviparous, and in these the lower part of the oviduct is enlarged and acts as a uterus. In male elasmobranchs the anterior part of the Müllerian duct persists. Paired intromittent organs (claspers) are developed on the pelvic fins of the males; these conduct the semen into the cloaca of the female.

In the teleostean and ganoid fishes (Teleostomi) the nephridial