When a baby grows up to be a millionaire or a bishop or a president, he is considered to have done so as the result of chance. Yet in none of these happenings does chance play so large a part as in the accident of birth. The selection of parents is most fortuitous; if your father and mother had not been who they were, you would not be you. Then, too, at the moment of conception any one of four hundred million male cells—spermatozoa—had an equal chance of becoming your biological father; only one did. And no two of these four hundred million spermatozoa were exactly alike. Each had a slightly different chromosomal makeup. The chromosomes are those physical-chemical constituents of the body cells which carry the blueprint of the offspring. When the two sets of blueprints—one from the father and one from the mother—are followed, a unique product results. In your case, by one chance in four hundred million, that unique product was you. If any other of those four hundred million spermatozoa had fertilized the ovum from which you were born, you certainly would have been a strikingly different person, perhaps of the opposite sex.
The complex, magic-seeming process of creating a baby is by now quite well understood. As it functions today, it is a far cry from the earliest beginnings of life on this planet.
The earliest life was probably a simple single-celled organism that reproduced by division into two smaller organisms. And when each half had grown to adult size, each of them divided into two. There were no special sex cells and no separate sexes. Some simple animals still adhere to this primitive reproductive pattern.
The Biological Advantage of Two Sexes
Later in the process of evolution separate sexes arose, and the fusion and mixing of two adult cells was required before their division into new young cells. The existence of separate sexes is of great advantage to both the survival and development of a species. That advantage lies in variability. If a single parent cell simply divides into two cells and each of these two cells into two cells, each of the progeny is an exact or almost exact duplicate of the original parent. Then if some adverse environmental influence comes along, such as drought or extreme cold, and the organism is especially susceptible to it, the whole species is wiped out. This is far less likely to happen if two parent cells fuse and their summation of materials then divides; for the progeny is not an exact duplicate of either parent, but has some characteristics of each. When this progeny fuses with an organism from a set of different parents, still greater variation results.
The process of the union of the sex cells, as it occurs in man and the other mammals, did not just happen; it evolved through many steps, some of which we can trace. More primitive forms of life, simple marine animals like the starfish, have a very wasteful form of sexual reproduction. There are two sexes, but the sperm and eggs are discharged haphazardly without any physical awareness or even propinquity between two parents. A more advanced stage in the evolution of the union of the sex cells is illustrated by fish. There is strong physical awareness between male and female fish, but absence of physical contact during mating. The male swims above the female, and as she discharges her eggs he discharges his sperm. In the frog, which has a still more advanced pattern of sexual behavior, there is not only sex awareness but actual physical contact. The male clasps the back of the female with a specialized clasp organ, and as she discharges her eggs he discharges his sperm upon them. All varieties of external insemination, however, are relatively wasteful and inefficient.
Internal insemination as practiced by man, by the other mammals, and by many non-mammalian forms, is far more efficient. In this pattern of reproduction a special organ of the male, the penis, is inserted into a special organ of the ‘ female, the vagina. In addition to depositing the semen well on the way to the precise area where it is to function, this mechanism of introducing the male ejaculatory organ deep within the body of the female protects the spermatozoa by releasing them in a highly favorable environment. The conditions of temperature, moisture, etc., within the cervical canal, the uterus, and the Fallopian tubes of the female reproductive tract are optimum for the spermatozoa.
The ageless, unhurried process of evolution has granted animals immense protection by making voluntary vital functions pleasurable. It is pleasant to eat, drink, void, defecate, sleep—and to impregnate. This pleasure in sex is termed libido, and the libido is created by body chemicals, sex. So closely interwoven are reproduction and sexual pleasure that the sex cells which unite to form the embryo and the sex hormones which create the appetite for mating are produced in one and the same organ—in the ovary of the female and in the testis of the male.
Mating among mammals may be restricted to a single annual season, as in deer, bears, and seals; or may take place in several isolated recurrent estrus periods (mating periods), as in cats, dogs, and the domesticated rodents, rats, mice, and rabbits. Still a third type of sexual rhythm is demonstrated by the primates—men, apes, and monkeys—a willingness to mate at all times without restriction of season or estrus period, though to be sure with unpredictable fluctuations of desire. In many women libido seems to bear a close relationship to the menstrual cycle, being most intense during the waning days of the period and immediately postmenstruum and pre-menstruum. This is difficult to understand from the viewpoint of survival of the species, since these three periods occur during the least fertile days of the month. Another oddity in the sexual behavior of primates is the pregnant female’s acceptance of the male. Among all other mammals copulation is rejected by the female throughout pregnancy.
The Sex Cells—Eggs and Spermatozoa
The eggs of all the higher mammals are similar in both size and appearance—round, with a clear, thin, shell-like capsule as rigid as stiff jelly. The capsule encloses liquid in which are suspended hundreds of fat droplets, protein substances, and other materials, including the nucleus. The egg, the largest cell of the whole body, is approximately 1/200 of an inch in diameter—about one-fourth as large as the period punctuating the end of this sentence. The eggs of rabbits, gorillas, dogs, whales, and pigs are all about the same size as human eggs. It is incredible but true that the whale’s tons and the mouse’s ounces spring from a round speck of matter with relatively the same diameter and weight.
The spermatozoa of different species show greater differences in form than do ova. The human spermatozoon consists of an oval head 1/6000 of an inch in diameter; most of the head is occupied by the nucleus. The head is attached by a short neck to a cylindrical middle piece, this in turn to a thin tail about ten times as long as the head. The tail is capable of rapid side-to-side lashing movements, by means of which the spermatozoon is propelled. The motor which moves the tail is located in the middle piece. A spermatozoon is 1/600 of an inch long from the top of its head to the end of its flagellum, a short, hairlike structure protruding from the end of the tail. Under ideal conditions a spermatozoon can swim an inch in eight minutes, but since its small size allows it to be easily blocked by the slightest obstruction, progress is seldom in a straight line, and frequently it takes much longer than eight minutes to accomplish an inch.
When ejaculated, spermatozoa are suspended in seminal plasma, a thick mucoid fluid produced by the male during sexual orgasm; it arises from the prostate, seminal vesicles and the accessory reproductive glands. The amount of semen ejaculated depends in part upon the interval since the previous ejaculation; in human beings the normal quantity varies from one-half to one and one-half teaspoons. In the stallion the quantity is ordinarily two ounces, ten to thirty times as much as in the human being. A fresh drop of semen seen under a microscope suggests the rush of traffic in a crowded city street. Myriads of spermatozoa dash here and there, now steering straight ahead, now halted by a speck of dust, now free again to scurry out of the microscope field. It is a spectacle surcharged with dramatic interest. The average human ejaculate contains almost a half billion spermatozoa in even suspension—a seemingly- extravagant superfluity of numbers, since only one spermatozoon fertilizes the egg.
The essential step in the initiation of a new life is fertilization, the penetration of the ovum by a spermatozoon and the fusion of parts of the two cells into a new single cell. From this united parent cell originate all the billions of cells which form the infant. The part of each cell which fuses is the nucleus, that glob of material in a cell which stains dark with aniline dyes in properly prepared microscopic slides. The nucleus is not a solid mass of tissue, as it may appear under low magnification, but is made up of a network of little dark-staining rods called chromosomes, which are referred to at the beginning of this article. There is a specific number of chromosomes characteristic for each species, and every cell of the body of each animal belonging to the particular animal family contains this number of chromosomes. The chromosomes are paired. In human cells there are forty-six single chromosomes, but each has a counterpart; thus there are twenty-three different pairs. Shortly before fertilization one chromosome of each pair is extruded from the nuclei of both the ovum and the sperm. Thus, when the two mature human sex cells fuse, each brings twenty-three chromo- somes to the process of fertilization, and by their union they restore the species number—in the case of man, forty-six. Research has shown that these minute microscopic rods are the all-powerful agents in the transmission of hereditary characteristics. To them each of us owes not only his sex but his body build, coloring, and, in large measure, his mentality, emotional makeup, and longevity.