The external generative organs have at first the same appear ance in the two sexes and con sist of a swelling, the genital eminence, in the ventral wall of the cloaca. This in the male be comes the penis and in the female the clitoris. Throughout the generative system the male organs depart most from the undif ferentiated type and in the case of the genital eminence two folds grow together and enclose the urogenital passage, thus mak ing the urethra perforate the penis, while in the female these two f olds remain separate as the labia minora. Sometimes in the male the f olds 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 horseshoelike 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.
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. Be fore the testis descends, a pouch of peritoneum called the proces sus 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 de scends, 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 gen eral cavity of the peritoneum disappears, and the part which re mains 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 since the gubernaculum becomes attached to the Miillerian 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 gubernacu lum running between it and the uterus remains as the liga ment 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.
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 dis tinct) 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 accountedfor 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 coelom, from which they pass into the large Miillerian ducts or oviducts. In the oviparous forms, such as the common dogfish (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 Miillerian duct per sists. Paired intromittent organs (claspers) are developed on the pelvic fins of the males; these conduct the semen into the cloaca of the female.