Animals and plants, apart from microscopic kinds of life, consist of enormous numbers of cells coordinated in various ways to form a single organism, and each consists of many different kinds of cells specialized for performing different functions. Certain tissues are set aside for the production of sexual reproductive cells, male or female as the case may be. Whether they are testes or ovaries or, as in some animals and plants, both together in the same parental individual, they are typically contained within the body, and therefore the sex cells usually need to be passed to the outside in order to function. Only in certain lowly creatures such as hydras is there a simpler state, for in hydras the testes and ovaries form in the outermost layer of cells of the slender, tubular body, and the sex cells when ripe burst directly from the simple, bulging gonads into the surrounding water. With few other exceptions, in all other creatures the gonads are part of the internal tissues and some means of exit is necessary. In some, such as most worms, all that is needed are small openings, or precisely placed pores, in the body wall through which sperm or eggs can escape. In most others, more is needed and a tubular sperm duct or an oviduct leads from each testis or ovary, through which the sex cells pass to the exterior. This is minimal equipment, except where none is needed. The gonad and its duct is accordingly comparable to other glands in the body; that is, the gland is generally a more or less compact mass of cells of a particular, specialized kind, together with a duct for passage of the product of the tissue to the site of action. Gonads secrete—i.e., produce and transmit—sex cells that usually act outside the body.

Differentiation between the sexes exists, therefore, as the primary difference represented by the distinction between eggs and sperm, by differences represented by nature of the reproductive glands and their associated structures, and lastly by differences, if any, between individuals possessing the male and female reproductive tissues, respectively.

Sex cells, sexual organs, other sexual structures, and sexual distinction between individuals constitute a series of evolutionary advances connected with various changes and persisting needs in the general evolution of animals and, to some degree, of plants as well. In other words, no matter how large or complex a creature may become, it still needs to deliver functional sex cells to the exterior. This condition is almost always the case for sperm cells. Among aquatic animals, particularly marine animals whose external medium, the ocean, is remarkably similar chemically to the internal body fluid medium of all animals, eggs are also in most cases shed to the exterior, where development of the fertilized eggs can proceed readily. Even so, time and place are important. Starfish, sea urchins, and many others, for instance, accumulate mature eggs and sperm in the oviducts and sperm ducts until an appropriate time when all can be shed at once. When one member of a group of such creatures begins to spawn, chemicals included in the discharge stimulate other members to do the same, so that a mass spawning takes place. One might say that the more they are together the more variable their offspring may be. This situation actually is the crux of the matter for nearly all forms of life, because while it may be possible for a single individual to possess both male and female gonads, producing both sperm and eggs, it remains generally desirable, if not essential, that eggs be fertilized by sperm produced by another individual. Cross-fertilization results in a much greater degree of variability than does self-fertilization. The existence of two types of individuals, male and female, is the common means of ensuring that cross-fertilization will be accomplished, since then nothing else is possible. Where the sexes are separate, therefore, all that is necessary is that members of the opposite sex get together at a time and place appropriate for the initial development of fertilized eggs. Typically, spawning of this sort is a communal affair, with many individuals of each sex discharging sex cells into the surrounding water. This process is only suitable, however, when eggs are without tough protective cases or membranes; that is, only when eggs are readily fertilizable for some time after being shed and while drifting in the sea. In this circumstance there is no need for individuals of the opposite sex to mate in pairs, nor is such mating practiced.

Mating

Mating between two individuals of the opposite sex becomes necessary when eggs must be fertilized at or before the time the eggs are shed. Whenever eggs have a protective envelope of any kind through which sperm cannot penetrate, fertilization must take place before the envelope is formed. The envelope may at first be a gluey liquid, which covers the egg and solidifies as a tough egg case, as in all crustaceans, insects, and related creatures. It may be a thick membrane of protein deposited around the egg, as in fishes generally; or it may be a material that swells up as a mass of jelly surrounding the eggs after the eggs have been shed, as in frogs and salamanders. And finally, it may be a calcified shell, as in birds and reptiles. In all of these organisms the sperm must reach the egg before the protective substance is added, except in those forms in which a small opening or pore persists in the egg membrane through which sperm can enter.

When and how such eggs need to be fertilized depends on the nature of the protective membranes and the time and place of their formation. The jelly surrounding frog and toad eggs, for instance, swells up immediately after the eggs are shed. Mating and fertilization must take place at the time of spawning. Male frogs mount the back of female frogs and each clasps his mate firmly around the body, which not only helps press the egg mass downward but brings the cloacal opening of male and female close together. Eggs and sperm are shed simultaneously, and the eggs are fertilized as they leave the female body. Fish eggs are also fertilized as or shortly after they are shed, although fish have no arms and mating generally is usually no more than a coming together of the two sexes side by side, so that simultaneous shedding of sperm and eggs can be accomplished. In other creatures the mating procedure may be much more complicated, depending on various circumstances. Crustaceans such as crabs and lobsters, for example, mate in somewhat the same manner as frogs, with the male holding on to the female by means of clawlike appendages and depositing sperm at the openings of the oviducts, which are typically situated near the middle of the undersurface of the body.