The gonads of the male are a pair of testes, each testis being attached to a kidney by connective tissue. The sperms produced by a testis make their way along tiny tubes to the kidney, passing through it but not through the excretory portion to leave by the sperm duct. This is also a urinary duct—sometimes, therefore, being termed a urinogenital duct. They are stored in a sperm sac until shed, whereupon they pass through the cloaca.
In the female the gonads are a pair of ovaries, diffuse thin-walled sacs showing immature eggs as black spots and attached to the dorsal surface of the body cavity by connective tissue. As the eggs ripen they break through the connective tissue into the body cavity itself, and eventually reach the funnel of an egg duct which carries the eggs to the exterior. A funnel is situated at the base of each lung. At first each duct is narrow and coiled, but farther back it is wider and has thicker walls which secrete the covering of jelly-around each egg. Near the rectum each oviduct widens out into an egg sac which retains the eggs until they are laid. It is the mass of eggs in thisthat gives the female a blown-out appearance during the breeding season. The two oviducts open into the cloaca just behind the apertures of the kidney ducts. The contraction of the wall of the cloaca forces out the eggs, aided probably by the pressure set up by the male in clasping the female.
The gametes produced by frogs are of two kinds and are utterly unlike each other in appearance and function. One type, commonly called the egg, is produced by the female frog and is therefore termed the female gamete. It is spherical, the size of a large pin ’s head, consisting of a nucleus embedded in cytoplasm which is black on top and white underneath ; this in turn is enclosed by a thin sheet of tough jelly-like material secreted by the gland cells of the oviducts. The male gametes produced by the male are called spermatozoa or, more commonly, sperms, which are far smaller than the eggs, the mass of 100,000 being equal to that of one female gamete. Each has a head containing the nucleus, with an end-piece and tail behind, the tail being a flagellum enabling it to swim when shed into water.
The pairing of the frogs occurs as previously described and after pairing the two parents then separate and the male usually dies. One sperm eventually penetrates the jelly of each egg and enters it, its nucleus fusing with that of the egg. Once this fusion has occurred, the membrane of the egg—now a zygote—changes in property so that no more sperms can enter. The yolk also changes to yellow, so that it is easy to tell which are eggs and which are zygotes in a mass of spawn. Should no sperm fuse with an egg, it fails to develop into a tadpole and eventually rots away. An infertile egg, then, is a female gamete, a fertile egg is a zygote : I.e. an egg is fertilized when conjugation occurs with a spermatozoon. We see here that conjugation of the gametes is preceded by a sexual act whereby the gametes are brought together. Incidentally, once the jelly round an egg has become fully swollen, no sperm can penetrate it and the egg remains unfertilized.