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| I | Introduction |
| II | Origin of the Reproductive Cells |
When the embryo of any sexually reproducing animal is undergoing cell division, certain cells, known as primordial germ cells, which are produced by such division, remain in an undifferentiated state. Cells other than primordial germ cells are known as vegetative cells or somatic cells; these cells become differentiated into tissues and organs. In invertebrates, the primordial germ cells congregate in the body cavity or in a section of the circulatory system; in vertebrates, these cells are located in organs that adjoin the excretory system. The tissues in which the germ cells lodge become reproductive organs known as gonads. These organs are embryologically derived from primitive kidneys located in the anterior, lateral part of the body; in most mammals, they shift before birth to the posterior, ventral region of the body. The primordial germ cells remain inactive in the gonads until the animal reaches sexual maturity, when the undifferentiated cells undergo a great number of normal cell divisions or mitoses. In the process of developing into mature reproductive cells or gametes, the germ cells undergo a special type of cell division, known as meiosis, which halves the number of chromosomes they carry. At the time of sexual maturity, the somatic cells composing the gonads of higher animals begin to secrete hormones that control the appearance of the various secondary sexual characteristics (see Sex).
| III | Gonads |
Organs that contain germ cells which later develop into male gametes or spermatozoa are known as testes or male gonads. Organs that contain germ cells which later develop into female gametes, eggs, or ova are known as ovaries. In many invertebrate species individual animals bear both testes and ovaries (see Hermaphroditism). In some invertebrates, and in most vertebrates, individuals bear either testes or ovaries, but not both sets of organs. In invertebrates, a single animal may have as many as 26 pairs of gonads; in vertebrates, the usual number is 2. Cyclostomes and most birds are unusual among vertebrates in possessing only a single gonad; owls, pigeons, hawks, and parrots are unusual among birds in having two gonads. The size of gonads increases at sexual maturity because of the great number of germ cells produced at that time; many germ cells are also produced during breeding seasons so that many animals have a seasonal increase in size of the gonads. During the breeding season of fish, the ovaries increase in size until they constitute about one-quarter to one-third of the total body weight.
The testes and ovaries of mature animals differ greatly in structure. The testes are composed of delicate convoluted tubules, known as seminiferous tubules, in which the primitive germ cells mature into spermatozoa. The testes of mammals are generally oval bodies, enclosed by a capsule of tough connective tissue. Projections from this tough capsule into the testis divide the testis into several compartments, each of which is filled with hundreds of seminiferous tubules. The mature spermatozoa are discharged through a number of ducts, called the efferent ducts, which communicate with the epididymis, a thick-walled, coiled duct in which the sperm are stored.
In all vertebrates below marsupials on the zoological scale, and in elephants, sea cows, and whales, the testis remains within the body cavity during the lifetime of the animals. In many mammals, such as rodents, bats, and members of the camel family, the testis remains within the body cavity during periods of quiescence, but moves into an external pocket of skin and muscle, known as the scrotum, during the breeding season. In marsupials, and in most higher mammals, including the human male, the testes are always enclosed in an external scrotum. During fetal life, the testes move through the muscles composing the posterior, ventral portion of the trunk and carry with them the portion of the peritoneum and skin surrounding these muscles. The channel in the muscles through which the testis moves is known as the inguinal canal; it usually closes after birth, but sometimes remains open and is then often the site of herniation (see Hernia). The portion of the peritoneum that the testis carries with it forms a double wall of membrane between the scrotum and testis and is known as the tunica vaginalis. Occasionally, the testes in the human male do not descend into the scrotal sac; this condition of nondescent, which is known as cryptorchidism, may result in sterility if not corrected by surgery or the administration of hormones. Retention of the testes within the body cavity subjects the germ cells to temperatures that are too high for their normal development; the descent of the testes into the scrotum in higher animals keeps the testes at optimum temperatures.
Unlike germ cells in the testis, female germ cells originate as single cells in the embryonic tissue that later develops into an ovary. At maturity, after the production of ova from the female germ cells, groups of ovary cells surrounding each ovum develop into “follicle cells” that secrete nutriment for the contained egg. As the ovum is prepared for release during the breeding season, the tissue surrounding the ovum hollows out and becomes filled with fluid and at the same time moves to the surface of the ovary; this mass of tissue, fluid, and ovum is known as a Graafian follicle. The ovary of the adult is merely a mass of glandular and connective tissue containing numerous Graafian follicles at various stages of maturity. When the Graafian follicle is completely mature, it bursts through the surface of the ovary, releasing the ovum, which is then ready for fertilization; the release of the ovum from the ovary is known as ovulation. The space formerly occupied by the Graafian follicle is filled by a blood clot known as the corpus hemorrhagicum; in four or five days this clot is replaced by a mass of yellow cells known as the corpus luteum, which secretes hormones playing an important part in preparation of the uterus for the reception of a fertilized ovum. If the ovum goes unfertilized, the corpus luteum is eventually replaced by scar tissue known as the corpus albicans. The ovary is located in the body cavity, attached to the peritoneum that lines this cavity.
The functioning of both male and female gonads is under the hormonal influence of the pituitary gland.
| IV | Transportation of The Reproductive Cells |
In male vertebrates, the ducts are directly connected to the testes. The male ducts include the epididymis, which lies attached to the testis, and which transports the male gametes to the vas deferens. The vas deferens carries the spermatozoa to the ejaculatory duct, the contractions of which cause the discharge of sperm into the posterior urethra.
In most fishes, the ovary has a hollow expansion through which the ova pass into the cloaca. In most other vertebrates, however, no direct connection exists between the ovary and the oviducts that carry the ova into the cloaca or into the independent external opening. In mammals, when the Graafian follicle bursts, the egg falls toward the interior of the abdominal cavity. The oviduct (known in higher mammals as the fallopian tube) has an open, funnel-shaped end located near the ovary, and the mature egg is drawn into the funnel by ciliary action. Occasionally, the egg misses the open end of the oviduct and falls into the abdominal cavity; such eggs are capable of being fertilized, resulting in ectopic pregnancies (see Pregnancy and Childbirth). In animals lower than marsupials the oviducts open directly into the cloaca; in marsupials and placental mammals, the oviducts, two of which are normally present, fuse at their cloacal ends to form a thick, muscular organ, the uterus or womb, in which the young develop, and a thinner channel, the vagina, which opens exteriorly.
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