The gonads are paired masses of mesodermal tissue (Fig. 382) that develop on either side of the mesentery in the antero-lateral part of the vertebrate body cavity. They become invaded by potential germ cells which, like all other cells, are the direct lineal descendants by mitosis of the original fertilized egg from which the organism arose (Fig. 383). They differ from other cells of the body in retaining their undifferentiated condition for a longer time.

Diagrammatic cross section across the Wolffian and genital ridges

Gonads that harbor future sperm cells are called testes, and those in which egg cells are embedded, ovaries. According to an almost universal rule among vertebrates, the testes and ovaries develop in different individuals, that is, the sexes are distinct from each other.

Separation of somatic and germplasmal cells in five stages in the developing embryo of the nematode worm, Ascaris

Usually gonads are massive structures that do not show metamerism, but in amphioxus they are arranged metamerically along the gill region in as many as twenty-six pairs. The largest pairs are in the middle with others decreasing in size at either end. Although superficially alike, the sexes are distinct.

In most vertebrates, however, there is not more than a single pair of gonads, and consequently metamerism or segmentation, so characteristic of many organs, disappears. In a few vertebrates only a single gonad is present, either as the result of the fusion of a pair as in the lamprey eel, or of unilateral degeneration, as in the hagfish, and the single left ovary of most birds, while frequently one of a pair of gonads will be smaller than its mate.

The germ cells, the distinctive features of gonads, arise from embryonic endoderm and migrate into the genital ridges.

The shape of gonads is in general influenced by the body form of the animal to which they belong. Thus, in the wormlike amphibian, Epicrinum, they resemble a segmented chain; in urodeles they are long and spindle-shaped, and in frogs and toads, oval. Among reptiles it is easy to distinguish the gonads of the short wide turtles from those of elongated snakes by shape alone.

During the breeding season gonads, particularly those of the prolific fishes, increase enormously in size with the multiplication and growth of the germ cells. The gonads of males are always more compact organs than those of the corresponding females because of the difference in size of the germ cells that are contained in them.


The testes (testis, witness) are so called because in former times hands were placed on these important organs when “testifying” under oath.

They not only harbor sperm cells but also produce internal secretions, or hormones, which are concerned with the development of the so-called secondary sexual characters that differentiate a male in appearance from a female.

A testis arises as a genital ridge along the ventromedial border of the mesonephros, or Wolffian body (Fig. 382). In most of the lower vertebrates it maintains an intimate relation with the anterior part of the mesonephros, that in this region loses its original excretory function, becoming transformed into a useful accessory reproductive organ, the epididymis, through which the sperm pass on their way to the sperm duct. Even higher up among mammals the epididymal portion of the transient mesonephros is still preserved as an integral part of the reproductive apparatus in the male. The entire structure becomes rudimentary in the female mammal, being represented only by useless fragmentary remains that, like degenerating organs generally, are frequently the focus for cystic formations and other pathological troubles.

Both blood vessels and nerves reach the testis between layers of tissue continuous with the peritoneum, thus forming the mesorchium, which serves as an anchoring bridge between the body wall and testis in the same way that the mesentery serves the intestine.

Teleost fishes form a notable exception with respect to intimate dependence of the reproductive organs upon the nephridial apparatus, for in them the testis is entirely emancipated from the mesonephros, with no attendant epididymis.

Diagram of the male sexual organs

The compact testes of mammals are generally somewhat oval bodies, enclosed in a close-fitting capsular sheath of connective tissue, the tunica albuginea, outside of which, on the free exposed surface at least, is a layer of mesothelium, the visceral layer of the tunica vaginalis, homologous with the peritoneum that lines the body cavity. Underneath the tough tunica albuginea is another layer of looser texture well supplied with blood vessels, the tunica vasculosa. Partitions of connective tissue extend from the tunica albuginea from one side to the other, dividing the whole testis into wedge-shaped compartments (Fig. 384) within which are lodged the seminiferous tubules that produce the sperm cells (Fig. 385). Two sorts of cells are found within the walls of these tubules; first, the supporting and possibly nutritive Sertoli cells; and second, the primordial germ cells, or spermatogonia, destined after repeated mitoses to give rise to the sperm.

Stages in the spermatogenesis of man, arranged in a composite to represent a portion of a seminiferous tubule

Between the tubules there are blood vessels, and still another kind of cells that form the endocrjne part of the structure, namely, interstitial cells, or the so-called “puberty glands” of Leydig.

In man the seminiferous tubules (Fig. 384), of which there may be several hundred, are very much kinked-up for the most part of their length (tubuli contorti), but they straighten out (tubuli recti) as they focus together at the inner side of the testis. A single convoluted tubule, occupying ordinarily a span of not more than one inch, stretches out when uncoiled to over a foot in length.

Tubuli contorti pass over continuously at their inner ends into tubuli recti which anastomose together into a network of tiny passages forming the rete testis, from which emerge other continuous tubules, ductuli efferentes, that pass through the epididymis, eventually reaching the sperm duct, or ductus deferens, leading to the outside.

The original anterior embryonic position of the testis within the body cavity is by no means always maintained. There is an evolutionary tendency, particularly among mammals, for the testes to migrate backward during the course of development. In the higher animals this tendency is carried to so great an extreme that the testes forsake the protection of the body cavity entirely and come to hang outside in a sac of outpushed skin and muscle known as the scrotum (Fig. 386).

Three stages in the descent of the testes

According to the degree of migration which the testes have undergone, animals in general may be separated into three groups as follows: first, those in which the testes remain within the confines of the body cavity; second, those in which there is a temporary descent during periods of sexual activity, followed by a withdrawal into the body cavity; and third, those with a permanent descent into the scrotum. To the first group belong all vertebrates below the mammals, and among mammals, monotremes, many insectivores, coneys, elephants, and whales. In the second group are many rodents, some insectivores, civet cats, otters, llamas, bats, camels, and certain apes. Those with a permanent descent of the testes after the fetal period, include marsupials, pinnipeds, most carnivores, ungulates, and primates.

The testes lie within a cavity in the scrotum which is homologous with the body cavity of which it was originally a part. As a result of this peculiar development there is a double layer of enwrapping peritoneum, or tunica vaginalis, about the testes, one the parietal layer lining the scrotal coelom itself, and the other the visceral layer which is reflected over the surface of the testes in close contact with the tunica albuginea, as already mentioned.

The inguinal canal, a continuous passage-way between the abdominal cavity and the scrotal sac, remains open in those mammals subject to a temporary descent of the testes. It is always pervious also in marsupials, although in other mammals having a permanent descent it ordinarily becomes closed. Through it runs the spermatic cord composed of arteries, veins, lymphatics, and nerves in addition to the ductus deferens itself. As in the case of all abdominal organs, the testis and these structures in the inguinal canal are all extra-coelomic, i.e., outside the coelom.

The wall of the scrotum is essentially the same as the neighboring body wall except that the skin is more highly pigmented and has a looser subcutaneous layer, and a greater supply of smooth muscle cells than the skin of other parts of the body.

In man the descent of the testes ordinarily occurs shortly before birth. Instances occur where the testes are abnormally retained within the body cavity, resulting in cryptorchism (cryptos, hidden; orchis, testis), but this unusual state of affairs has a perfect explanation in the light of comparative anatomy. The tapir and rhinoceros have no pendulous scrotum, although the testes leave the body cavity and come to lie in pockets close under the skin. A similar anomalous condition is seen in some flatfishes (Pleuronectidae), in which not only the testes but also the ovaries are crowded out of the cramped quarters of the body cavity and find a haven in pockets “extending backwards into a kind of concealed scrotum between the integument and muscles on each side above the anal fin.” (Owen.)

Why the testes of the highest vertebrates, with their precious contents, should push out the body wall into a scrotal sac and hang over the edge of the pelvic bones in so exposed a situation is not at once apparent. Certainly the outpushing of the body wall by the protruding testes leaves centers of structural weakness that entail liability to hernia and frequent attendant troubles. Studies by Moore and others seem to indicate that temperature as high as that normally maintained within the mammalian body is somewhat detrimental to the functioning of spermatozoa. The descent into the scrotal sac is a device whereby the testes are exposed to a lower temperature than that within the body cavity, thus favoring the development of the sperm. In the case of animals having a temporary descent of the testes there is delayed development of the sperm while the testes are retained within the body cavity. This theory also accounts for the frequent sterility that accompanies cryptorchism. It does not explain the production of sperm by avian testes, located in the body cavities of animals maintaining higher bodily temperatures than do mammals.


Ovaries, like testes, arise as genital ridges and are “glands” of double function, producing cells, or ova, and endocrine secretions that are distributed through the blood. They may be simply attached to the peritoneum, or encapsuled when they hang freely in the body cavity, being connected to the body wall by a mesovarium, homologous with the mesorchium of the testes. They may also be embedded in the peritoneum, as in the rabbit and cat.

Asymmetrical development of the ovaries appears in some elasmobranchs, particularly of the families Trygonidae and Myliobatidae, where the right ovary is forced to make way for the intestine with its extra large bulky spiral valve. Some of the bony fishes, for example the perch Perea, and the curious little sand-lance Ammodytes, show a fusion of the two ovaries into one, while in Morrnyrus oxyrhynchus of the Nile, and some others, only the left ovary develops.

In birds also only a single ovary develops (Fig. 387), the left one being the one that is “left,” although a few birds, such as owls, hawks, pigeons, and parrots sometimes show remains of the lost right ovary.

Urogenital organs of a female bird showing the single left ovary from the dorsal side

The advantage resulting from the single ovary in birds may have to do with the drastic elimination of unnecessary ballast in adaptation to flight, or possibly with the safe manipulation of large eggs with breakable shells. Only a single oviduct remains when there is a single ovary, but in the case of two ovaries and two oviducts it might well prove disastrous if two eggs with fragile breakable shells should approach the narrow exit side by side at the same time.

Ovaries of elasmobranchs, crocodiles, turtles, birds, and mammals are more or less solid in character, and the eggs that are embedded in them dehisce through the periphery directly into the body cavity to be picked up by the open ends of the oviducts.

Amphibians, lizards, and snakes have saccular ovaries which are hollow within, but the eggs still break through the outer walls, arriving in the oviducts by the indirect coelomic route.

The ovaries of many teleost fishes, that are likewise hollow, form an apparent exception. In this case the eggs collect within the ovaries and pass directly into an ovarian sac from which an oviduct conducts them to the outside without their entering the body cavity at all. As a matter of fact the space within the ovaries and that within the sac are both actually shut-off portions of the original body cavity (Figs. 388 and 389). The ovarian sac, formed when the ventral free edge of the genital ridge grows laterally to join the body wall and close off a portion of the body cavity, extends posteriorly to attach to the anterior end of the short oviduct and provide an independent communication between the sac and the outside. In several families of teleosts, in which these sacs do not develop, the eggs are shed into the main body cavity from which the oviducts carry them to the outside. In all of these cases it is clear that the eggs are inducted into the body cavity on the way to the oviduct.

Diagrammatic sections showing development of closed ovaries. Diagrams of female urogenital ducts in teleosts

The garpike, Lepidosteus, alone of the ganoid fishes follows the teleost plan. Other ganoids have open oviducts and “solid” ovaries.

During the breeding season the ovaries of fishes become engorged with eggs and increase enormously in size, until they may equal as much as twenty-five or thirty per cent of the total body weight.

The cavity of the saccular amphibian ovaries is lymphoid in character and is not comparable with that of teleosts from which it differs fundamentally in origin, its lumen having been hollowed out secondarily instead of representing a portion of imprisoned body cavity.

In turtles the ovaries are symmetrically disposed, but asymmetry in position appears in lizards and becomes quite pronounced in snakes, with the right ovary placed in advance of the left.

The ovaries of the duckbill, Ornithorhynchus, owing to the few large projecting eggs which they contain, are somewhat lobulated, “resembling a bunch of grapes,” while those of most other mammals are relatively small, round, and smooth in contour, being enveloped in a dense layer of fibrous tissue, the tunica albuginea. Underneath this layer is the cortex, which is the general seat of the ova, while the interior tissue constitutes the medulla, or vascular region of the ovary.


By far the greatest number of all vertebrates are dioecious (di, two; oikos, house), that is, the ovaries and testes are borne by different individuals. There are a few instances, however, of hermaphroditism, when the two sexes (Hermes and Aphrodite) are combined in a single individual, a condition not uncommon among invertebrates, that by reason of isolation or absence of locomotor organs need to overcome the difficulties attendant upon pairing. For example, the sea bass, Serranus scriba, is regularly hermaphroditic and self-fertilizing, while another exceptional teleost, Chrysophrys auratus, is reported as a protandrous (prot-, first; andr-, male ) hermaphrodite, being “first male” and afterwards female.

The unpaired gonad of the hagfish Myxine, appears at times to produce sperm, and at other times eggs, while the larval ammocoetes form of the lamprey eel, according to Lubosch, shows hermaphroditic gonads in twenty-five per cent of the cases.

Occasional hermaphroditism has been observed in codfish, mackerel, and herring among bony fishes, as well as in the Alpine newt, Triton, and in frogs and toads among amphibians.

Instances of so-called human hermaphrodites, cited in medical literature, usually refer to abnormalities of the external genitalia rather than to functional gonads. It is extremely doubtful whether there is any authentic case on record of a human being that has produced both eggs and sperm, since the evidence in supposed instances tends to be extremely obscure.


The fertilized egg of any vertebrate, when ready to give rise to a new individual, is composed of germplasm laden with the hereditary potentialities of the species to which it belongs. By a succession of innumerable mitoses this original cell soon becomes a mass of daughter cells from which the somatoplasm, or the body of the individual, is elaborated. Somewhere early in the series of cell divisions there comes a parting of the ways between reserve germplasm and somatoplasm, whereby one of the two cells resulting from a mitotic division is set aside to carry on the traditions of the race, while the other continues in the service of the developing individual (Fig. 383). Each of these two kinds of ancestral cells, particularly that of the somatoplasmic line, undergoes countless subsequent mitoses, with the result that the two lines come to differ fundamentally according to the purpose for which they are destined.

The germplasmal cells find sanctuary in the gonads where they remain practically unchanged until puberty, that is, until the time when the individual that holds them in trust arrives at sexual maturity. They then awaken into activity, following the prolonged dormant period, and by a series of transforming changes become mature sperm and eggs, fit for union into a zygote or fertilized egg. This process of preparation is called gametogenesis (gamete, marrying cell; genesis, origin).

The essential process in the final differentiation of the sexual germ cells is the elimination of half of the chromosomal material bearing the hereditary potentialities from the nucleus of the egg or sperm, so that upon the union of the two germ cells the number of chromosomal units proper and characteristic for the species in question will be restored. This maturation process has been described in Chapter VI. If this reduction of the chromosomes did not occur before the fertilization of the egg, there would be a doubling of chromosomes in each generation which would inevitably upset the necessary machinery of mitosis.

The transformation of spermatogonia, or primordial sperm cells, into mature locomotor sperm having half the original chromosomal contents occurs within the testes in the walls of the tubuli contorti (Figs. 384 and 385), the origin of which must be sought in the embryo.

The embryonic genital ridge on either side between the mesonephros and the mesentery within the body cavity is composed of a mass of mesodermal cells that form the testis. The cells on the outside of this testicular mass facing the body cavity are arranged in a layer of epithelium which becomes invaded by the primordial germ cells.

By repeated mitoses of cells in this superficial layer, cordlike cellular masses push down into the undifferentiated cell mass of the testes. These solid cellular strands later become hollowed out and enormously elongated to form the different tubuli of the testes, in the walls of which are lodged the spermatogonia.

The ovary develops from the genital ridge in the female with a similar outer stratification of epithelium in which are certain germplasmal cells that in mammals proceed to proliferate down into the substance of the ovary as cellular masses, called Pfluger’s cords (Fig. 390). These cords, however, do not hollow out in the manner of the corresponding structures in the testes, but instead form follicles. Each follicle is characterized by the presence of a central primary cell, or ovum, surrounded by numerous follicular cells, which later may be sacrificed as nutriment for the cannibalistic egg cell within.

Oogenesis. The genesis of the ovum, within the Graafian follicle in the ovary, is indicated by the successive numbers

In mammals the follicular mass, as Graafian follicle, becomes hollowed out and filled with fluid so that the developing ovum is practically surrounded by a double wall of follicular cells, like a lighthouse on a peninsula that projects into an inland sea.

The Graafian follicle migrates as a whole from its embedded position and gradually crowds to the surface of the ovary where it bulges out as the fluid-filled cavity increases in size. Eventually the wall of the follicle ruptures and the egg is extruded into the body cavity.

The cavity of the ruptured Graafian follicle immediately becomes filled with a blood clot, the corpus haemorrhagicum, which in turn is gradually invaded and replaced by peculiar yellow lutein cells, making the corpus luteum. This periodically recurring mass of cells has an important endocrine function, that will be referred to again in the following chapter.

Eventually the corpus luteum is obliterated by an invasion of connective tissue, leaving a scar on the surface of the ovary, the corpus albicans, which is all that remains to tell the story of the origin of a mammalian egg. Successive phases of the development of the Graafian follicle in the mammalian ovary are shown in Figure 390.