Neurosecretions formed in the brain in response to environmental stimuli regulate the synthesis and release of hormones known as gonadotropins, which, in turn, stimulate the gonads. Cyclical intervals of illumination (photoperiods) may be the principal environmental factor regulating gonadal activity. Although cyclical temperature changes are experienced by many species, as are fluctuations in food supply, rainfall, and salinity, their precise effects and those of many other stimuli, independently or in combination, have not yet been defined for any species. Photoperiodicity, temperature, and perhaps all other cycles are attributable to the seasons, and to the 24-hour day.

As a result of rhythmic stimulation by gonadotropins secreted by the pituitary gland, the gonads grow, mature, and produce gametes and hormones. Certain of these hormones, known as androgens, are thought to be produced chiefly by interstitial cells and are more abundant in males. Hormones known as estrogens are probably produced chiefly by ovarian follicles and their thecas. Circulating progestins are produced in greatest quantities by corpora lutea. Although the gonadal hormones of different species vary somewhat in structure, their effects are essentially the same. As the quantity of pituitary gonadotropins decreases, the activity of the gonads slows and may temporarily cease.

The effects of gonadal hormones may be summarized as follows:

Gonadal hormones induce growth of and maintain the cyclical function of the reproductive tracts, accessory sex glands, and copulatory or ovipository organs. They thereby provide for the storage, nutrition, and transport of gametes; the secretion of necessary substances onto the surface of gametes; and the ultimate extrusion of sperm, eggs, or the products of conception. In mammals, therefore, they prepare the vagina for copulation and the uterus for implantation of eggs; in addition, gonadal hormones maintain pregnancy until birth or until placental hormones can take over their function. The hormonal basis for the maintenance of viviparity in vertebrates below mammals is almost unknown.

Gonadal hormones participate in the maturation of gametes still in the gonads by augmenting the metabolic effects of other hormones.

Gonadal hormones are essential for the differentiation of many secondary sex characters—the physical differences between the sexes—facilitate amplexus (copulatory embrace) and provide for the protection or nutrition of young. Secondary sex characters include scent glands; sexually linked pigmentation of the skin or its appendages; the nature of any vocal apparatus; hardened areas on the appendages that facilitate amplexus; distribution of hair; body size; mammary gland development; and other features.

Gonadal hormones participate in the induction of behaviour necessary for the union of sperm and eggs; this includes migratory phenomena, heat (estrus) in mammals, courtship, territorial defense, mating, and care of eggs or young.

Gonadal hormones participate in a mechanism that affects the pituitary, thereby imposing certain restraints on the secretion of gonadotropins.

The effects of a cyclical environment on gonads is illustrated in mammals that ovulate spontaneously. Ovulation is induced by ovulatory hormones released rhythmically from the pituitary gland. Newborn mice maintained during the first week of life in regular, natural photoperiods will, on reaching maturity, ovulate regularly. Newborn mice kept in continuous light during this interval will not ovulate regularly. The photoperiods in which these animals live as neonates, or newborn, establish an intrinsic brain rhythm that subsequently results in cyclical reproductive activity. If mature female mice that have been ovulating regularly are subjected to continuous light, ovulation ultimately becomes arrhythmical. This suggests that the rhythmical environment is the ultimate regulator of the gonads. Because of the effects of cyclical photoperiods, spontaneous ovulation occurs about the same time of day or night in all members of species intensively studied thus far. Golden hamsters ovulate shortly after midnight; chickens and Japanese rice fish ovulate in the morning. Not all mammals ovulate spontaneously, however. In those that do not (e.g., reflex ovulators), including some cats, rodents, weasels, shrews, rabbits, the act of mating substitutes for the environmental effects on the pituitary gland in releasing ovulatory hormones (see hormone).

Provisions for the developing embryo

Among the requirements of developing embryos are nutrients, oxygen, a site in which to discharge metabolic wastes, and protection from the environment. These needs exist whether the embryo is developing outside the body of the female parent (oviparity), or within, so that she delivers living young (viviparity). Combinations of yolk, albumen, jellies, and shells contributed by the female parent, as well as membranes constructed from the tissues of the embryo meet the embryo’s needs.

Oviparous eggs are usually supplied with enough nutrients to last until the new individual is able to obtain food from the environment. The alternative, postnatal parental feeding, is uncommon. Oviparous animals that develop from yolk-laden eggs are not hatched until they resemble adults. Those that develop from eggs with moderate amounts of yolk hatch sooner, usually into free-living larvae; in this case the larvae transform, or undergo metamorphosis, into adults. The eggs of amphioxus, an oviparous protochordate, contain almost no nutrients; the embryos hatch in an extremely undeveloped but self-sustaining state as few as eight hours after fertilization. The yolk mass is large in some animals and becomes surrounded by a membrane called the yolk sac, the vessels of which convey yolk to the embryo. In some species, yolk also passes from the yolk sac directly into the fetal intestine.

Oviparous fishes and amphibians develop in an aquatic environment, and exchange of oxygen and carbon dioxide and elimination of metabolic wastes occur through the egg membranes. Oviparous reptiles, birds, and monotremes develop on land, and gaseous exchange is accomplished by two membranes (allantois, chorion) applied closely to the shell. The allantois also receives some wastes. Drying out or mechanical injury of embryos of reptiles, birds, and mammals is prevented by still another membrane, the amnion, which is a fluid-filled sac immediately surrounding the embryo.

Viviparity has evolved in some members of all vertebrate classes except birds. When eggs heavily laden with yolk and surrounded by a well-formed shell develop within the female, the parent may provide the developing young only with shelter and oxygen (ovoviviparity). At the opposite extreme, if eggs contain only enough nutrients to supply energy for a few cell divisions after fertilization, the female provides shelter, oxygen, and nourishment, and, in addition, excretes all metabolic wastes produced by the developing organism (euviviparity). Between these extremes are numerous intermediate degrees of dependence on the parent.

Teleosts have evolved many unusual adaptations for viviparity. In some viviparous teleosts the eggs are fertilized in the ovarian follicle, where development occurs. The granulosa cells either form a membrane that secretes nutrients and assists in respiratory and excretory functions or they may be ingested along with follicular fluid, nearby eggs, and other ovarian tissue. A common site for development is the ovarian cavity, which may become distended with as many as nine series of embryos of different ages. Embryos in this location are bathed with nutritive fluids secreted by the epithelium of the cavity. In some species, mortality rates of intraovarian young are high, and surviving individuals ingest those that die. In still other species, extensions of villi in the ovarian lining invade the mouth and opercular (gill) openings of the embryo, filling the opercular chamber, mouth, and pharynx with surfaces that secrete nutrients. The embryos also develop specialized surfaces for nutrition, respiration, and excretion. An enlarged pericardial (heart) sac or an expansion of the hindgut of the embryo may occur next to the blood-vessel containing (vascular) follicular wall. Vascular extensions may grow out of the anus, urinogenital pore, or gills of the embryo. Other embryonic surfaces—including ventral body wall, fins, and tail—may participate in the support of viviparity. These embryonic surfaces may lie in contact with the follicular or ovarian epithelium, or they may simply be bathed by ovarian fluids. One or more combinations of the maternal and embryonic specializations described above, as well as many others, make viviparity possible among teleost fishes. In a number of teleosts the eggs are incubated, or brooded, in the mouth of the male for periods as long as 80 days. The oral epithelium becomes vascular and highly glandular. In seahorses and pipefish the female deposits her eggs in a ventral brood pouch of the male, and the embryos develop there.

In viviparous elasmobranchs development takes place in the uterus, the lining of which develops parallel ridges or folds covered with villi or papillae (trophonemata) that constitute a simple placenta (site of fetal–maternal contact). In contact with this region is the yolk sac of the embryo, which serves as a respiratory and nutritive membrane. Trophonemata secrete uterine fluids that supplement the yolk as a source of energy. In one shark (Pteroplatea micrura), trophonemata extend into the spiracular chamber (an opening for the passage of respiratory water) of the young and secrete nutrients into the fetal gut. In another (Mustelus antarcticus), the uterine folds form fluid-filled compartments for each embryo. The yolk sac may lie in contact with the uterine lining, or projections of the sac may extend into uterine pits. When the stored yolk is used up before birth, the yolk sac may serve for the absorption of nutrients; i.e., as a placenta. In a few species, immature eggs that enter the oviduct are eaten by the developing young.

Very few amphibians bear living young. In the viviparous frog Nectophrynoides, all development, including larval stages, occurs in the uteri and the young are born fully metamorphosed; i.e., except for size they resemble adults. N. occidentalis, an African species, has a nine-month gestation period. There is almost no yolk in the egg and no placenta, so it is probable that uterine fluids provide nourishment and oxygen. In N. vivipara there are as many as 100 larvae in the uteri, each with long vascular tails that may function as respiratory membranes. Gastrotheca marsupiata is an ovoviviparous anuran with a gestation period of three to four months. In certain viviparous salamanders the extent of the nutritional dependence on the mother varies. After depleting their own yolk supply, the larvae of some forms eat other embryos and blood that escapes from the uterine lining. Conventional viviparity is rare among amphibians; however, they have evolved unusual alternatives. In some anurans the young develop in such places as around the legs of the male (Alytes), or in pouches in the skin of the back (some females of the genera Nototrema, Protopipa, and Pipa). In Pipa, vascular partitions in the skin pouch separate developing young, and the larvae have vascular tails that absorb substances. In Nototrema larval gills have vascular extensions with a similar function. The male Chilean toad (Rhinoderma darwinii) carries developing eggs in the vocal sac until the young frogs emerge.

Some snakes and lizards and all mammals except monotremes exhibit viviparity to some degree. The same extra-embryonic membranes found in oviparous reptiles and mammals (yolk sac, chorioallantoic membrane, amnion) function in viviparous ones. Here, the extra-embryonic membranes lie against the uterine lining instead of against an egg shell. At special sites of fetal–maternal contact (placentas), viviparous young receive oxygen and give up carbon dioxide; metabolic wastes are transferred to maternal fluids and tissues; and, in euviviparous species, the young receive all their nutrients. Yolk-sac placentas are common in marsupials with short gestation periods (opossum, kangaroo) and in lizards. Chorioallantoic placentas (i.e., a large chorion fused with a large allantois) occur in certain lizards, in marsupials with long gestation periods, and in mammals above marsupials. The yolk-sac placenta does not invade maternal tissues, but intimate interlocking folds may occur between the two. The chorioallantoic membranes of reptiles and mammals exhibit many degrees of intimacy with maternal tissues, from simple contact to a deeply rooted condition (deciduate placentas). Chorioallantoic or chorionic placentas represent specializations in a chorionic sac surrounding the embryo. The entire surface of the sac may serve as a placenta (diffuse placenta, as in pigs); numerous separate patches of placental thickenings may develop (cotyledonary placenta, as in sheep); a thickened placental band may develop at the equator of the chorionic sac (zonary placenta, as in cats); or there may be a single oval patch of placental tissue (discoidal placenta, as in higher primates).