article : Reproductive System in Life organism (2)

V

Genitals

In animals that lay their eggs and discharge their sperm into water, the spermatozoa reach the eggs by chemical attraction; the eggs of individuals of a species attract only the sperm of members of the same species. When eggs and sperm are deposited at great distances from each other, the number of eggs fertilized is small. Many amphibians and aquatic vertebrates solve this problem by attaching themselves to their mates by means of holdfast mechanisms, such as claspers; when the female of these animals deposits the eggs, the male can immediately deposit sperm in the same location.

In terrestrial animals, various adaptations have been developed whereby internal fertilization of the eggs may be produced. The male snake, which discharges the spermatozoa through a cloaca, has, for example, anal hooks that are inserted into the cloaca of the female during the breeding season. These hooks bind the male and the female together while the spermatozoa are being discharged. External reproductive organs used by animals to facilitate internal fertilization are known as genitals or genitalia.

The male genital of all mammals above the monotremes is the penis, a protrusible, erectile organ that directs the spermatozoa into the female cloaca or vagina. In turtles and crocodiles, which are the most primitive animals possessing the organ, the penis is located on the ventral wall of the cloaca and is grooved along its upper side. The spermatozoa travel along the groove into the female cloaca. In marsupials and placental mammals, the penis is a closed tube, composed of three bundles of tissue, bound together by connective tissue and covered with loose skin. Two large bundles of tissue, known as the corpora cavernosa, constitute the upper portion of the penis; these bundles contain numerous compartments that are filled with blood during sexual excitement, causing the penis to become stiff and erect. The flow of blood into the corpora cavernosa is controlled by sacral spinal nerves. Below and between the corpora cavernosa is the third bundle of tissue, which is known as the corpus spongiosum; this bundle is perforated by the urethra and, in several lower mammals, also contains a bone that serves to further stiffen the male genital. At the terminal end of the penis is a sensitive cap known as the glans; in marsupials the glans is forked. In many mammals, the male genital, when not erect, is withdrawn into a sheath in the body. In primates, including the human male, the penis is pendant, and the glans is covered with a layer of retractile skin, known as the prepuce or foreskin, which corresponds to the sheath of lower animals.

The chief female genital is known as the vagina. This organ is present in all marsupials and placental mammals. In marsupials, two vaginas and two uteri are present; in primates, including the human female, only one vagina is present; and in mammals intermediate in development between the marsupials and primates, various degrees of partially united, double vaginas are present. The external end of the vagina is covered in virgin primates by a membrane, known as the hymen. Anterior to the hymen is the external opening of the urethra. The urethra and the opening of the vagina are contained in an indented space which is known as the vestibule. In primates, two membranous folds, known as the labia minora, are found on either side of the vestibule. In primates, including the human female, two additional folds, the labia majora, enclose the labia minora. The clitoris, at the front of the labia, is homologous with the penis although greatly reduced in size (see Homology below).

VI

Accessory Glands

Among the glands accessory to the reproductive process are those that provide a fluid medium in which the spermatozoa may live, those that produce mucus which reduces friction during copulation, those that emit alluring odors to members of the opposite sex, and those that secrete nourishment for the ova, the embryos, and the newborn young.

The seminal vesicles of the male have already been mentioned as organs that secrete mucus. The most important male accessory gland is the prostate gland, a compound gland about the size of a chestnut located at the base of the urethra where the urethra leaves the bladder and enters the penis. The prostate secretes a thin milky fluid with a characteristic odor; this fluid constitutes the greater part of the semen that is deposited in the female vagina and that contains the spermatozoa. The prostate gland is present only in placental mammals, and among these mammals is absent in edentates, martens, otters, and badgers. Cowper's glands are two small glands, about the size of a pea, located on each side of the base of the penis. Their secretion is thick and clear and is believed to protect the spermatozoa against excess vaginal acidity. Cowper's glands are absent in bears, dogs, and aquatic mammals.

The primary lubricating glands of the female are the cervical glands, located in the uterus where the uterus communicates with the vagina, and Bartholin's glands, located in the vestibule between the hymen and the labia minora. Both sets of glands secrete mucus. Female placental mammals also have uterine glands that prepare the uterus for the reception of the fertilized egg.

The anal glands of many mammals secrete special substances called pheromones, which signal reproductive readiness by scent to members of the opposite sex. Pheromones may occur in other glandular secretions as well.

Among the different structures serving to nourish the young, the placenta of placental mammals is unique (see Fetus). The mammary glands of mammals are also included among accessory reproductive glands (see Breast). Female egg-laying animals have albumen glands, which coat the zygote with nutrient albumen before the egg is laid, and nidamental glands, which surround the zygote and albumen with a leathery or calcareous shell.

VII

Homology

The sex of a young embryo is indistinguishable, males and females passing through similar embryonic stages. Embryo males and females develop almost a duplicate set of reproductive organs, one set becoming vestigal shortly before birth, and the other set becoming prominent. Most cases of so-called mammalian hermaphroditism are actually cases of abnormal development in which external genitalia similar to those of both sexes have been developed. For example, mammalian females have a small, erectile organ, consisting of two corpora cavernosa, located in the upper portion of the vestibule. This organ, called the clitoris, is homologous with the male penis; except in lemurs and a few rodents it does not contain the urethra, which is usually located beneath the clitoris. In species in which the male possesses a penis bone, the clitoris of the female contains a small bone.

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