For conception to occur it is necessary that a sperm cell make contact with and penetrate an egg cell. Where are the two sex cells produced, and how is contact between them established?
The Testes and Sperm Production
Throughout a man’s reproductive life, which in rare cases continues after the age of eighty or ninety, spermatozoa are constantly being formed by the two testicles suspended in the scrotum, a thin-walled sac of skin. The scrotum is a highly specialized structure which, because of its external location and its large area of skin surface for heat evaporation, constantly maintains the testicles in an environmental tempera- ture several degrees below that of the interior of the body. The sperm-making cells within the testicles are extremely sensitive to heat and when subjected to temperatures as high as the body’s interior they rapidly degenerate and cease producing the male sex cells.
In early fetal life the testicles are situated high up within the abdomen, and they gradually migrate downward, reaching the scrotum about eight weeks before birth. Occasionally boys delivered at full term are born with undescended testicles; in some, the situation corrects itself within the first few months or years of life. If lack of descent persists, at five or six years of age the child is given injections of the gonadotrophic hormone recovered from the urine of pregnant women, or tablets, by mouth, of the male hormone, testosterone, first isolated from the urine of bulls. Either stimulates the testicles to grow, and because of the increased weight they may then drop spontaneously into the scrotum. Cases in which this does not occur require surgical correction; at operation the testicle is guided into the scrotum and sewn in place. The ideal time to perform such an operation is between the ages of nine and eleven years. Otherwise, by the time of full adolescence, the sperm-producing tissue of the testicle is already so irreparably damaged by chronic exposure to the relatively high intra-abdominal temperature that reposition has uncertain value. Another evidence of the sensitivity of sperm-producing cells to increased temperature is the temporary infertility in normal males which often follows bouts of high fever or the temporary immersion of the scrotum in water of 130 degrees Fahrenheit.
The Journey of Spermatozoa Up the Male Ducts
From each testicle the spermatozoa slowly pass upward through a long, narrow, much coiled tube, the epididymis, which empties into a wide, straight tube, the vas deferens. The trip through this maze of ducts requires more than fifteen days, the spermatozoa maturing as the journey progresses. If the male is continent for some time, the spermatozoa which complete the journey are dammed up in the out-ter end of the ducts and suffer effects of senility. Therefore, the first ejaculation after a long interval may show impaired motility in most cells. For this reason, doctors prescribe moderately frequent intercourse for most couples desirous of pregnancy who are having difficulty in conceiving.
Spermatozoa do not make their own way up the male ducts, since they are motionless at this stage, but are propelled upward by imperceptible contractions of the muscular tissue forming the tube walls. It is only after the mass of sperm cells is diluted during orgasm by the addition of fluids from the prostate and other male glands that they are thrown into vigorous movement. Spermatozoa remain actively motile in the seminal fluid at room temperature for twenty-four hours or longer after ejaculation and as long as thirty-six to forty-eight hours in the upper reaches of the female reproductive tract. Spermatozoa can be kept viable indefinitely by the addition of a glycerine compound to the semen, then rapid freezing and storing at a constant, very low temperature. When thawed even after months or years, the cells become normally motile and if used in artificial insemination are fully capable of causing pregnancy. The social implications of this are enormous.
Function and Structure of Male Sex Organs
We must briefly consider the functions and structure of the reproductive organs to understand the process through which sperm and egg are brought into contact. The male genital organs serve two primary functions in the act of reproduction—to manufacture spermatozoa and to deposit them in the body of the female.
The penis, the male depository organ, has a conical end, the glans, which in the uncircumcised male is protected by a thin, elastic, retractile skin cover, the foreskin. The urethra, the tube from which the urine ordinarily flows and from which during orgasm semen is ejaculated, runs through the center of the shaft of the penis and terminates in a small ellipitcal opening at the tip of the glans. When relaxed, the penis of the average man measures about four inches in total length and has a diameter of about an inch. When completely erect under the stimulus of sexual excitement, it measures slightly more than six inches and is an inch and one-half in diameter. Erection is accomplished by the rapid inflow of blood into the special spongy tissue which forms the organ, and by the temporary imprisonment within it of this blood under pressure—which comes about through the closure of exit valves in the veins which circulate blood through the penis. Orgasm consists of a series of muscular contractions involving the whole male tract, which drives out the semen in spurts, after which the valves open in the veins, releasing the imprisoned blood, allowing the penis to soften and wilt.
Function and Structure of Female Sex Organs
The reproductive organs of the female, however, serve a fourfold purpose. First, they provide a receptacle for the male semen; second, they produce the ovum; third, they serve as a trysting site for sperm and egg; and fourth, they furnish a place where the fertilized ovum can develop into a.
The vagina is the body cavity adapted to the reception of the semen. In the virgin, its entrance is, in most cases, partly closed by the hymen, a skinlike membrane stretching across the vaginal entrance, containing one or more small openings. Normally the membrane is destroyed at the initial sexual intercourse. However, the hymen is highly variable, being absent at birth in some cases and complete in others; therefore, caution is necessary in attaching medico-legal importance to its condition as evidence of virginity. At the outer end of the vagina, just above the urethra from which the urine exits, is the clitoris, the homologue of (that is roughly, but not exactly, corresponding to) the male’s penis. When unerected, it is usually about a half-inch in length, and when erected, almost twice as long. It has no function except to react to sexual stimulation and enhance the female’s pleasure and response. The two ovaries, almond-sized organs which lie in the lower part of the abdominal cavity, produce the ova. Ordinarily, one ovum matures each month from the onset of the menses until the menopause, except during the nine months of pregnancy and a few months thereafter. Since there is but one ovum a month, only one of the ovaries matures an egg every four weeks, the two ovaries dividing the task with no apparent plan. Sometimes they alternate; at other times the same ovary produces the ovum several months in succession. If one ovary is surgically removed, the surviving one takes over the complete burden of egg production, maturing an ovum each month, usually with no reduction in fertility. The two Fallopian tubes, or oviducts, one on either side of the uterus, lead from the abdominal cavity near each ovary to the interior of the uterus. They form the pathway for the upward trek of the spermatozoa and the downward journey of the ovum. Finally, there is the pear-sized, pear-shaped, muscular uterus, enclosing a slitlike cavity. It is here that the fertilized ovum embeds and the fetus develops.
After this introduction we are now prepared to discuss the time, the place, and the manner in which sperm and egg meet.
The ripe egg is attached to the interior of a half-inch, blister-like structure, the Graafian follicle, which bulges from the surface of one ovary. Through some mechanism not fully understood, the follicle bursts, spilling its fluid contents and the tiny egg cell directly into the cuplike opening of the partially erect Fallopian tube, or into the abdominal cavity near the vicinity of the tube. The tube then acts like a siphon and sucks the spilled egg into it. This monthly delivery of an egg from the ovary—ovulation—is totally independent of sexual intercourse, occurring with equal frequency in the sexually developed virgin and the married woman. If fertilization does not occur—obviously its occurrence is relatively infrequent—the tiny unfertilized egg simply dies, crumbles into even tinier pieces, which the woman’s body liquefies and absorbs.
To be fruitful, sexual intercourse must take place within forty-eight hours or less of the occurrence of ovulation. If we can place ovulation in a definite time relationship to some easily observed recurrent phenomenon of the reproductive cycle, such as menstruation, we are well on the track of knowing when sexual intercourse is most likely to result in pregnancy. Significant data on ovulation have been collected by several methods—by the examination of ovaries at the operating table and in surgically removed specimens; by the actual washing of an egg from the Fallopian tube; by observation of pregnancies following a single, accurately dated copulation; and by observation of results from artificial insemination, when the wife of a sterile husband is impregnated by mechanical injection into the vagina (or occasionally the uterus) of a single specimen of fertile semen, obtained from the husband, or if he is inadequate, from another man, termed a donor. From such observations it appears that ovulation most often occurs between eight and nineteen days after the onset of the menses, the exact day being influenced by the length of the menstrual interval. Women with short menstrual intervals—for example, twenty-five days—are likely to ovulate early, and those with long intervals—such as thirty-one to thirty-five days—late. Actually, a woman ovulates about fourteen days before the onset of her next menstrual period. However, when the egg is fertilized and she becomes pregnant there is no next menstrual period until many weeks after her confinement, ovulation having occurred about fourteen days before she would have menstruated, had she not been impregnated.
Since most women menstruate approximately every twenty-eight days, if one counts the first menstrual day as day one, the usual time of ovulation is day thirteen or day fourteen, which explains the fact that impregnation is most likely to occur in mid-cycle. Impregnation, however, in the very rare instance may result from intercourse at virtually any time during the menstrual month—which implies that ovulation in exceptional cycles occurs at exceptional times. The periods when impregnation is least likely, the relatively ‘safe periods’ for sex relations without causing conception, are the first week of the cycle and the last week, the week prior to menstruation; the smallest number of conceptions takes place at these times.
The Egg’s Journey Down the Tube
After ovulation, the egg, having passed from an ovary into the Fallopian tube, travels down the tube, which is three to five inches long in the human female. The passage takes from sixty to seventy-two hours. The walls of the tube encircle a narrow canal with a bore as small as a broomstraw. The mechanism that propels the egg downward through the tube toward the uterus seems to be a combination of fluid currents flowing down and up the tube, and rhythmic muscular contractions, like those which carry food and excreta through the intestinal tract. When the ovum is spurted from a ripe ovarian follicle, it is surrounded by a thick, loosely adherent covering of small cells. Many of these are gradually brushed loose by contact of the egg with the sides of the tube, especially contact with the ciliated cells of the tubal wall, which have hairlike protrusions that beat back and forth, cleaning off most of the cells surrounding the ovum.
The Upward Journey of the Spermatozoa
The mid-portion of the tube is the rendezvous for egg and sperm. Explanations of how spermatozoa ascend from the vagina into the uterus, and from the uterus to the meeting place in the tube, have shifted as knowledge of the subject has increased and clarified. A hundred years ago a spermatozoon was believed to be endowed with instinctive, bloodhound-like qualities which directed it along the proper path to insure fertilization.
Today it is known that the fate of the several hundred million spermatozoa depends in part on the phase of the recipient’s menstrual cycle.
During the three or four days before ovulation and the day of ovulation itself, the canal of the cervix, the entrance passage into the uterus from the vagina, is filled with a profuse, transparent, watery mucus which furnishes a highly favorable environment for sperm cells and through which they swim with ease. The appearance of this profuse mucus explains why some women notice a colorless vaginal discharge each month during the mid-cycle days. Some women occasionally stain or even bleed lightly for forty-eight hours in mid-month, this being synchronous with the time of ovulation. Many have pain for four or five hours in one side or the other of the lower abdomen, depending on whether the egg that particular month was ovulated from the right or left ovary. At the other times of the month the cervical canal contains a scant, sticky, opaque mucus, much less easily penetrated by spermatozoa: many of them are entrapped and halted in it like flics on flypaper.
During intercourse the spermatozoa are catapulted into the upper vagina, into the region of the cervix. When ejaculated, the semen immediately becomes semi-gelatinous, but soon after becomes liquefied again. The sperm cells swim haphazardly in all directions, some into the upper recesses of the vagina, some toward the outside, others away from the middle of the vagina far to one side or the other. The bulk of the spermatozoa never reach the protective confines of the cervical canal, but remain in the vagina, exposed to the hostile environment of vaginal secretions, which are normally quite acid in reaction. Sperm cells are sensitive to an acid medium, and those remaining in the vagina become motionless and dead within a few hours. A relatively few by sheer spatial accident immediately gain the sanctuary of the cervical mucus. This was demonstrated by some recent studies in which cooperating couples notified the physician as soon as male orgasm had been accomplished. The physician then took samples of mucus from high up in the cervical canal. Much to the surprise of the scientific community the cervical mucus tested was already swarming with sperm cells. The cervical mucus is as alkaline as the blood, and some of the spermatozoa swim straight up the one-inch mucus-filled canal with almost purposeful success, while others bog down on the way, getting hopelessly stranded in little tissue bays and coves. A small proportion of the total number ejaculated eventually reach the cavity of the uterus and begin their upward two-inch excursion through its length. Whether this progress results solely from the swimming efforts of the spermatozoa or whether they are aided by fluid currents and muscular contractions of the uterus is not known. The undaunted ones, those not stranded in this veritable everglade, reach the openings into the uterus of the two Fallopian tubes and continue their journey upward through the tubes.
If the egg is discharged from the right ovary and has reached the mid-portion of the right tube, about two inches above the uterus, a spermatozoon swimming up the left tube has, of course, no chance of impregnating it. It is calculated that only about 2000 of the 400,000,000 cells ejaculated ever reach the trysting site, the mid-segment of the Fallopian tube containing the egg. The one sperm of the 2000 that achieves its destiny has won against gigantic odds, several hundred million to one. The baby it engenders has a far greater chance of becoming president than the sperm had of becoming part of a baby. No one knows just what selective forces are responsible for the victory. Perhaps the winner had the strongest constitution; perhaps it was the swiftest swimmer of all the contestants entered in the race. Perhaps it was merely the luckiest in finding a fluid current leading straight to the ovum. According to experimental evidence, this total five-inch journey requires approximately thirty or forty minutes. If ovulation occurred within several minutes to several hours before the sperm’s journey’s end, the ovum will be in the tube, awaiting fertilization; if ovulation took place at a time more remote, the egg cell will have already begun to deteriorate and fragment, rendering it incapable of fertilization by the time the spermatozoon reaches it. On the other hand, if ovulation has not yet occurred, but takes place within thirty-six hours after intercourse, living spermatozoa will be cruising in the tube, waiting for the egg.
We have followed the sperm and egg to their meeting place, and we can now observe what happens when they meet— that is, the actual process of fertilization.
The Process of Fertilization
The sperm has several roles in fertilization. The sperm head carries a chemical substance, an enzyme, hyaluronidase. It dissolves the glutinous bond by which some of the small follicle cells adhere to the surface of the ovum—those which the tubal cilia had not brushed off. It has been determined experimentally in rabbits that a few spermatozoa are insufficient to accomplish fertilization, although only one actually penetrates the egg capsule. The reason for the necessity of an excess number at the fertilization site is probably that sufficient hyaluronidase may be released to denude the egg of the cells still adhering to its surface. By combining its chromosomes with those of the ovum, the spermatozoon endows the offspring with half of its heredity—all it inherits through the father. The eggs of some simple animals are at times normally activated to full development without the aid of sperm, and eggs of other species can be artificially activated by certain physical and chemical agents. This has not been done in higher mammals. To date there is no authentic case of a child’s having been conceived who lacked a biological father, despite contrary suggestions in recent English yellow journals.
Penetration of the Ovum by a Spermatozoon
The egg capsule, the zona pellucida, is relatively firm and rigid; its thickness is approximately one-tenth the diameter of the round egg cell. Precisely how a spermatozoon penetrates this wall is not known. In mammals there are no special openings in the egg capsule, nor does the capsule soften and flow about the snerm head, engulfing and sucking it into the interior of the egg cell—as the process in some marine worms has been described. It appears that the sperm makes head-on contact with the egg capsule and by its own swimming prowess bores through it. In the rat, the whole spermatozoon enters, the lashing tail as well as the head. In this and probably most other animal species, the interior of the egg is very sticky, and forward movement of the fertilizing spermatozoon is not continuous. Following penetration, the sperm progresses to the pole of the egg opposite its point of entry and there comes to rest, the tail ceasing its thrashing movements. Additional spermatozoa have been observed to enter the rat egg after the fertilizing spermatozoon had already assumed its final position and become quiescent. These additional spermatozoa remain in the narrow fluid space between the inside of the egg capsule and the jelly-like interior substance, taking no part in fertilization. It is not known whether the penetration of the rat ovum by several spermatozoa occurs only in eggs fertilized under the microscope, or whether it also happens in nature. In the human being, it is not known whether the head alone or the whole spermatozoon, including the tail, enters the egg, nor do we know whether more than one spermatozoon ever penetrates the human ovum.
Fusion of the Two Nuclei
In the rat during the first nineteen hours after fertiliza- tion both the male and female nuclei enlarge and migrate toward the center of the cell. The next step is the fusion of the two nuclei into one parent nucleus. When this has been accomplished, fertilization is completed, and the fertilized ovum then begins to divide into two cells; the two cells divide into four, the four into eight, and so on, creating after twenty-one days a newborn rat weighing less than an ounce. In the human being the same process, after 266 days, creates a seven-and-a-half-pound baby. Whether rat or human, the infant animal is formed by a marvelously intricate and orderly arrangement of billions of cells.
The Early Hours of the Fertilized Egg
In the mouse the two-cell egg appears twenty-four hours after fertilization, and the four-cell stage after thirty-eight to fifty hours. In the human a two-cell stage was observed thirty-six hours after fertilization and a twelve-cell development in seventy-two hours.
In all mammals the developing egg in its earliest phase, while still a traveler down the Fallopian tube, is a solid mass of cells, aptly termed a morula (from morum, Latin for ‘mulberry’). It is still round and has increased little if at all in diameter, its contents merely having divided into smaller units. In this respect it is like a real-estate development. At first a road bisects the whole area; then a cross road divides it into quarters, and later other roads into eighths and twelfths. This happens without the addition of any land, simply by subdivision of the original tract.
The egg passes from the tube into the uterus on the third or fourth day, when it is no longer a solid mass of cells, the cells having arranged themselves about the outer surface of the sphere, the center now being occupied by fluid. It is then termed a blastocyst (Greek, ‘sprouting bladder’). By the fifth day the original single large cell has subdivided into sixty small cells and floats about the slitlike uterine cavity a day or two longer, then adheres to its inner lining, which has previously been prepared by a special hormone or chemical, progesterone, fed by the ovary into the bloodstream. This hormone has made the lining succulent and swollen with a network of new and enlarged blood channels coursing through it. The developing egg, possessing an enzyme or chemical which digests away the surface cells to which it adhered, then sinks down into the depths of the uterine lining. The process is something like hoeing the firm, dry earth above to plant the corn in the soft, moist, rich earth beneath. By the twelfth day the human egg is already firmly implanted, but the damaged, superficial uterine cells through which it passed have only partially healed to cover over its outer surface.
On microscopic study the twelve-day egg already shows a specialized accumulation of cells which later will form the embryo. The remaining cells become the afterbirth and membranes.
Impregnation is now completed, as yet unbeknown to the woman. She has not even had time to miss her first menstrual period, and other symptoms suggestive of pregnancy are still several days distant.