THE EMBRYO
The Development of Form—The Determination of Sex—Twins—The Rate of Growth—The Newborn Infant—Heredity—Maternal Impressions.
The new human being begins existence, as I have shown, as soon as the ovum is fertilized, though at that moment it consists merely of a solitary cell formed by the union of the two parental cells. From a beginning relatively simple the human body develops into the most complex of living structures; and, startling as it may appear to be, it is demonstrably true that every one of the millions of cells which compose an adult has descended from the ovum. Furthermore, the individual himself is not the entire progeny of the ovum; the placenta and the membranes dealt with in the preceding chapter, we saw, were also derived from that same source. They possess only a transitory importance, to be sure, and to most persons they are less interesting than the embryo, yet we gave them consideration before discussing its growth because the manner in which the ovum becomes attached to the womb and draws nutriment from the mother primarily determines the fate of a pregnancy.
Now that we have become familiar with the arrangements for the protection of the embryo, we are prepared to learn how it develops, and may accept the phrase, embryonic development, to cover the whole period of existence within the womb. In a more technical sense, however, the use of the term embryo is limited to the first six weeks of pregnancy and designates the condition of the young creature before it has acquired the form and the organs of the infant; after that time the unborn child is called a fetus. Embryonic development, therefore, in the strictest sense of the term, chiefly involves the shifting of various groups of cells and the bestowal upon them of different kinds of activity. During this period comparatively slight growth takes place. By about the twentieth week, the house, it may be said, is set in order; and there follows a period marked by the rapid growth of the fetus.
THE DEVELOPMENT OF FORM.—A very old explanation of embryonic development was that the process consisted altogether in growth. According to that view the embryo lay curled up in the egg; at the outset it was equipped with organs, limbs, features, and all the other bodily structures found in an adult. In order that the ovum might be transformed into a mature infant, only unfolding and growth were required. After the microscope came into use, however, so simple an explanation could no longer be accepted. Scientists soon realized that the embryo did not exist "ready made" in the ovum, which, even when magnified, failed to bear the faintest likeness to a human being.
Although the microscope made impossible this very simple explanation, it gave in return a truer, if more complex, account of the transformation from egg to offspring. By this means it has been definitely proved that the ovum multiplies rapidly after it has been fertilized, and becomes, as was explained in the preceding chapter, a sac-like structure within which hangs a tiny clump of tissue. This inner mass of cells forms the embryo.
It has proved a difficult task to secure very young human embryos, and many of the ideas we hold relative to the initial stages in the development of man are based upon what has been found true in certain mammals, the class of animals to which we belong. The youngest human ovum known at present has already undergone about two weeks' development, and there the embryo is represented by a flat disk. From this stage to the stage of complete development a satisfactory series of embryos has now been collected, but it is impossible to give here, even in outline, a description of the evolution of the human embryo. No one can understand this intricate subject without the aid of diagrams, models, and other material beyond the reach of all save laboratory workers.
By the end of the second month the development of the embryo has advanced so far that anyone could recognize its human shape. About that time, too, the external sexual organs make their appearance. At first these are quite similar in both sexes; and, if they are used as the criterion, it is possible only toward the end of the third month to say whether the embryo is a male or female.
THE DETERMINATION OF SEX.—The fact that a number of months pass before the sex can be distinguished by an external examination of the fetus has led to the erroneous belief that it can be influenced during the early part of pregnancy or actually determined at will. Various means to accomplish this have been suggested; many of them depend upon modifying the mother's mode of living according as a boy or girl is desired. The most widely known of these doctrines, that of Schenck, was to the effect that the sex of the offspring is always that of the weaker parent. He suggested, therefore, that increasing the vigor of the mother by an appropriate diet would produce a male child, whereas a decrease in her strength would lead to the opposite result. His views, however, were incorrect. After studying extensive statistics Newcomb came to the conclusion that "it is in the highest degree unlikely that there is any way by which a parent can affect the sex of his or her offspring."
Moreover, the results of experimental research clearly indicate that we shall never possess the means by which a mother may control the sex of her child. In the main laboratory investigations have sought to answer two questions. First, at what time is the sex of the offspring determined? and, second, what accounts for the origin of a male in one instance and of a female in another? The study of these problems has been carried on chiefly in connection with insects, worms, and fowl; but as yet insurmountable difficulties have prevented similar investigations in higher animals. For this reason, it is not without the greatest caution that results thus far obtained may be assumed to apply to man.
Sufficient facts, however, have been collected to admit no doubt regarding the answer to the first question. In most animals it is definitely known that the sex of the offspring has been fixed when the male cell enters the female cell, in other words, at the instant the ovum is fertilized. Excellent reasons exist for believing that human beings conform to this rule, and that the sex of the child is unalterably determined at the moment conception occurs. Consequently, any attempt to influence it after that event must prove futile.
For the present, the second question cannot be answered with equal assurance. More than five hundred theories have been offered to explain the relation of sex; nearly all of them have no reasonable foundation and are only of historical interest. The view that girls are derived from the right ovary, boys from the left, has long since been disproven, and deserves mention merely because the laity still believe it. Happily, during the last few years, observations and experiments have been made which greatly advance our knowledge of the subject and give promise of an early solution of the problem. The controlling factor in sex determination has been narrowed down to three possibilities; it is inherited either from the single cell contributed by the father or from the single cell contributed by the mother, or it is determined by the effect these two cells have upon each other at the moment when they unite. In most animal species the weight of authority distinctly favors placing the whole responsibility upon the male cell.
According to recent evidence, there are two kinds of male germinal cells; one kind giving rise to female offspring and the other to male. In all probability, at the time of the marital relation, these varieties are deposited in the vagina in equal numbers; and, moreover, the mode of their production is such as to place absolutely beyond human control the possibility of changing this ratio. Since only one spermatozoon enters the ovum, whether or not the child will be a boy or a girl depends entirely upon which type gains entrance. If this explanation is correct, and it is in accord with careful biological observations, it removes from the mother all responsibility for the sex of her child. Furthermore, since the facts indicate that male-producing and female-producing spermatozoa are present in equal numbers, it follows that practically there is an even chance that an embryo will develop into a boy or a girl.
Birth statistics bear out this conclusion, as data gathered from many countries indicate that when long periods of time are studied 105 boys are born with a surprising regularity for every 100 girls. Thus, the records of Berlin, Germany, for a hundred years show that the maximum difference occurred in 1820, when the males outnumbered the females by 4.79 per cent.; the minimum difference, which was noted in 1835, was .64 per cent. in favor of boys.
No inquiry is more often submitted to the physician by prospective mothers than this, "Can you tell me if my baby will be a boy or a girl?" He cannot. Many rules, to be sure, have been advocated as safe guides toward reaching the correct answer; every midwife possesses her individual formula which she has "never known to fail." But the boastful success depends upon the application of some such method as the following, which I have heard my teacher, Dr. J. Whitridge Williams, expose to his classes. The patient is asked if a boy or girl is desired. She confesses, and is then informed that the sex of her child will be the opposite of her wish. When this guess proves to be correct, there is no doubt of the prophet's wisdom; when it is not, his honor is protected, for the parents have had their hope fulfilled. Their happiness makes them forgetful that the guess was wrong, or, for that matter, that it was ever made.
It was once believed that the sexes might be distinguished before birth by the number of heart beats occurring within a minute. In a general way, the action of this organ in females is somewhat more rapid than in males; and so it was thought that a rate of 144 or more indicated the female and a rate of 124 or less the male sex. But experience has taught that this rule leads to accurate prophecy in no more than half of the cases. As a matter of fact, no means of definitely foretelling the sex of the child has been discovered, and I doubt if it ever can be.
TWINS.—As every one knows, pregnancy commonly terminates with the birth of a single child. Twins appear in approximately only one of ninety pregnancies, while triplets are extremely rare. It is true that even quintuplets may occur, though up to 1904 only 29 authentic instances could be collected from the whole range of medical literature.
Twins are most frequently born to parents whose ancestors have established this tendency; the trait is usually inherited from the mother's family, though occasionally it is passed on through the father. Of course, that does not explain the cause of twins, which in reality may result from either of two circumstances. More commonly their genesis depends upon the ripening of two eggs at about the same time and the fertilization of both by two different spermatozoa. The children, in this instance known as double ovum twins, may be of the same sex or not. On the other hand, single ovum, or identical, twins are always of the same sex; this follows, since but one egg and but one spermatozoon are here concerned. The incident permitting twins to develop from a solitary ovum must occur soon after conception has taken place. It will be remembered that the first step in the development of the fertilized ovum consists in its dividing into two cells. Ordinarily, both these take part in the development of one embryo, but occasionally they separate and give rise to two. Frequently, the presence of twins can be recognized during the latter months of pregnancy, and accurate means are known of determining after they are born to which variety any given pair belongs.
THE RATE OF GROWTH.—When we recall the definite and often marked differences in the physical character of women, such as weight and height, it is surprising to learn that the prenatal development of their children proceeds with uniform speed. One very practical result is that the physician is thus enabled, at the birth of a premature infant, to estimate accurately the period of its development. Various criteria, some of which are easy of application, aid in this determination. For example, the length of the child is practically constant for each of the ten lunar months into which the whole gestation period is divided; if, therefore, the length of the newborn infant is known, the stage of its development can always be inferred. From the fifth month the calculation is especially simple, since the length measured in centimeters divided by the figure 5 gives the month to which pregnancy has advanced. Similarly, we can infer the period of development from the weight, though the calculation is more intricate and the method less reliable, inasmuch as the size of the child in the latter months varies somewhat according to the weight of its mother.
At the end of the fifth month, the weight of the fetus is from nine to ten ounces; whereas an average infant when born at the expiration of the full term of pregnancy, that is, with the completion of the tenth month, weighs about seven pounds. The fetus, therefore, acquires roundly ninety per cent, of its weight during the second half of pregnancy, which clearly indicates that Nature reserves this period of gestation for the fetus to increase in size, a phenomenon less mysterious but no less important than the evolution of the embryo.
Nothing is more valuable than the weight in affording an indication as to whether a prematurely born infant may be reared. It is unusual to raise a child weighing less than four pounds, which corresponds approximately to the end of the eighth lunar month of development (a trifle more than the seventh calendar month). After this time, the prospect of living becomes greater in proportion to the nearness with which the infant has approached maturity. No truth exists in the widespread belief that the seventh-month child is favored above that born later but before the natural end of pregnancy. Experience has taught that the probability of success in rearing the child increases rapidly after the seventh month. This is reasonable on the following somewhat theoretical grounds. The digestive organs later attain a higher state of perfection, and are better prepared to carry on their work satisfactorily. Moreover, the gradual deposition of fat beneath the skin during the last two months of pregnancy materially assists in fitting the child for the conditions met with in the external world, since the fat affords a barrier against the escape of heat generated within the body, making it much easier to keep the child's temperature at the normal point. Even other more technical reasons could be given to demonstrate the error of the superstition regarding the seventh-month child—a conviction endorsed by medical men hundreds of years ago and as yet not discarded by the laity.
When pregnancy has reached "term," the child, having completed its prenatal development, is ready to cope with conditions as they exist in the external world. At term the average child is twenty inches long and weighs 7 1/7 pounds (3,250 grams). The length is remarkably constant; but the weight, as is well known, is often somewhat above or below the average figure. In a general way, smaller children occur in the first than in subsequent pregnancies, and, moreover, may be expected when the mother is a small woman, or poorly nourished, or has worked hard during her pregnancy. On the other hand, a tendency to bear large children is present when the opposite conditions prevail. It is not unusual to see infants weighing eight or nine pounds at birth, but babies of more than ten pounds are rare, and the fabulous, though not infrequent, reports of fifteen and twenty-pound infants are probably not based upon actual weighings, but upon the impression of someone who has merely seen the child or perhaps guessed the weight from lifting it.
Although the fetus frequently changes its position during the earlier months of pregnancy, generally by the beginning of the tenth lunar month it has assumed a permanent posture. It has then reached such a size that it can best be accommodated in the cavity of the uterus if its various parts are folded together so as to give the fetus an ovoid shape. To secure this form its back is arched forward, and its head bent so that its chin touches its chest; its arms are crossed just below the head, its legs raised in front of the abdomen, and its knees doubled up. In this form the fetus occupies the smallest possible space.
With relation to the mother the position of the child, for several weeks before birth, is one in which its long axis is parallel to the long axis of her body. This remains true no matter whether the head or the buttocks are to precede at the time of birth. In ninety-seven out of a hundred cases, however, the head lies lowermost and consequently is the first portion of the child to be born. The opposite position, in which the head is the last portion born, is, even with the most skillful treatment, somewhat more serious for the infant, though not for the mother.
THE NEWBORN INFANT.—The baby at birth is not a miniature man. As compared with an adult its head and abdomen are relatively large, its chest relatively small; its limbs are short in proportion to the body; and at first glance it appears to have no neck at all. The middle point of a baby's length is situated about the level of the navel, whereas in a man the legs alone represent approximately half his height. The changes after birth consist chiefly in growth; but not altogether, since at least one organ, the thymus gland, becomes smaller and completely disappears during childhood, and other organs, especially the liver, are proportionately smaller in the adult than in the infant.
The body of the infant also differs from that of the man in possessing greater softness and flexibility. These qualities depend upon the nature of its skeleton, which is composed of more bones than later in life, when several have fused together to form one to give the mature body a more rigid frame. Furthermore, the individual bones are not so firm, consisting of an elastic material called cartilage, so that some movements which in an adult would cause such serious injuries as fractures and dislocations are perfectly harmless to a newborn child.
The legs are not only short in proportion to the body but are always curved, and the feet are held with the soles directed toward one another, a position clearly abnormal in the adult. But every mother should know that these are natural conditions in the infant, and are the result of the posture of the child before birth. They soon straighten out. The bowed legs of an adult are of an entirely different origin, resulting from a disturbance of nutrition in infancy called rickets.
A small amount of short wooly hair is usually found over the back of a newborn infant. More conspicuous, however, is the presence there of a gray, fatty substance which, though always more abundant over the back, is at times distributed over the whole body; rarely is it entirely absent. The material, technically named the vernix, is the product of the glands in the skin and is a perfectly normal secretion. After its removal, which is readily accomplished by greasing the infant with lard or vaselin before giving the initial bath, it never reappears.
A varying amount of hair covers the head of the infant. No significance should be attached to the quantity, for the conviction that exists, especially among negroes, that a heavy suit of hair for the child occasions "heart-burn" in the mother during pregnancy is without foundation. The color of the hair at birth does not indicate its ultimate shade; changes are often noted during infancy. Similarly the permanent color of the eyes is not assumed until later; at the time of birth the eyes are generally, if not always, blue in color.
A baby's head is a matter of great concern to the family. Occasionally, the skull is round and well shaped from the moment of birth, but more often it is long and narrow; sometimes the form is even startling to the inexperienced. The peculiar shape of the head results, of course, from its passage through the birth-canal and is not a sign of any disease. In a few weeks, or even less, the strange appearance passes away. It is unwise to attempt to alter the shape of the head by bandaging or massaging since the growth of the brain will spontaneously accomplish what is desired; interference can do no good, and may do serious harm.
Nature facilitates an appropriate molding of the head during birth so as to permit its easy passage through the bony pelvic cavity of the mother, and gains that end in two ways. The bones of the head remain pliable until after the infant is born, and, further, their edges are not welded together as in an adult, but are separated from one another by an appreciable distance. During the act of birth the edges are brought into contact or even overlap, materially reducing the size of the head. Within a few hours after birth the bones again spread apart, and some months elapse before they begin to unite; the union is not completed until some time during the second year of infancy.
Many mothers are anxious to know how far the senses of the infant have developed when it enters the world. This problem has stimulated some scientific investigation, though hardly so much as its interest would justify. Two lines of inquiry have been pursued toward its solution. The objective point of one of these has been to determine how nearly the sense organs of the newborn correspond anatomically to those of an adult; that is how perfectly has their organization been completed. The other has been to learn how the infant reacts when the various senses are stimulated; the interpretation of these reactions is, however, particularly liable to error and sometimes amounts only to guesswork.
The organization of the nerves and muscles in the eye is far from perfect at the time of birth. The muscles act irregularly; indeed, the lack of muscular adjustment is such that movements of the eye likely to alarm the parents are regularly observed in very young infants. Furthermore they cannot focus images which fall upon their eyes. The retina, which receives visual impressions, has reached such development at birth, however, that sensations of light can be perceived. For example, if a lamp is suddenly flashed before the face of a newly born baby it cries. From this and similar evidence, indicating that strong light irritates the delicate structures of the eye, we have learned that a nursery should not be illuminated, during the day or night, so brightly as the rooms adults occupy. Certainly several weeks, and probably several months, pass before an infant can see anything save as blurs of light and darkness. Objects, such as a hand, probably appear as shadows, which are not correctly interpreted until late in infancy.
In regard to color vision we have as yet no reliable information concerning children under two years of age. Infants of less than a year have been known to distinguish certain colored papers. But such discrimination is probably due to a difference in brightness of the colors.
Although the organ of hearing is well developed at birth, the drum of the ear in very young infants cannot transmit sounds, as in the adult. For the latter kind of transmission it is necessary that the pressure on both sides of the drum-membrane should be equal, and this is arranged by the admission of air to the middle ear through a passage from the throat. At the time of birth, on account of the swollen condition of the mucous membrane which lines this passage, it is blocked, and the middle ear is filled with fluid; these conditions interfere with the transmission of sound, and consequently its perception is dulled. But even in the absence of a drum-membrane an adult can hear; the vibrations in such cases are transmitted through the bones of the skull, and this very likely also occurs in newly born infants. In most instances, at least, they react to a disagreeable noise within the first twenty-four hours, and their sensitiveness in this direction explains why the nursery should be kept quiet.
Investigators have not come to uniform conclusions concerning the sense of smell and of taste. In all likelihood, smell is not acute at the time of birth. Taste probably is better perceived, yet some newborn babies are said to suck a two per cent solution of quinin as eagerly as milk, though stronger solutions are distasteful. According to the best available information a young infant can detect the difference between a sweet, bitter, sour, or salty taste only when the tests are made with a solution possessing the quality in question to a marked degree. It is common knowledge that babies cheerfully suck the most tasteless objects, and it is not improbable that at first the reaction depends upon the temperature of the object and the feeling it creates in the mouth.
The moment it is born, a baby perceives pressure if its skin is touched. To this sensation, however, some parts of the body are much more sensitive than others; the tongue and lips are most sensitive of all. Heat and cold are probably perceived more acutely by infants than by adults; to pain, on the other hand, babies are less sensitive. An infant is aware of the movements of its own muscles, and also appreciates a change from one position to another, as experienced nurses know very well, and on that account carefully avoid keeping a baby on one side continuously.
The vast majority of movements performed by young infants are reflex acts, that is, the cerebrum, the part of the brain with which thinking is done, is not concerned with their performance. Of these reflexes the most notable are sucking and swallowing, but sneezing, coughing, choking, and hiccoughing may also be observed; stretching and yawning have been recorded in several instances, even during the first days of infant life. None of these movements, we must remember, are produced consciously; the baby cannot reason and does not recognize anyone, even its mother.
HEREDITY.—The transmission of bodily resemblance and of traits of character from parent to child is a broad and complicated subject, whose fundamental principles biologists are just beginning to grasp. The facts thus far established regarding heredity relate chiefly to plants and to the lower animals. There is no doubt whatever that the meager knowledge we possess of heredity in man will be amplified and will ultimately indicate on the one hand the marriages which are advisable and, on the other hand, those which are not. Indeed, the foundations for a science called Eugenics, which purposes to improve the human race in this way, have already been laid. It is barely a decade, however, since our knowledge of heredity has approached that order and system which entitle it to be ranked as a science; and in this brief period great strides could hardly be expected in its most intricate field, that of human inheritance.
The modern teachings of heredity are of interest to us, nevertheless, since they intimate the time when a child's inheritance is fixed and the means by which hereditary characters are conveyed. To understand these fundamental points we must recall that at the moment of conception a male germinal cell combines with a female cell, and that this act, which is named fertilization, brings together vital elements from the two parents. We have seen that the spermatozoon represents the solitary contribution of the father toward the development of the child, and the spermatozoon, therefore, must convey the material basis of paternal inheritance. Similarly we might expect the ovum to be the bearer of the maternal qualities inherited by the child. This is actually true; but much of the evidence is of a technical character and must be omitted. Yet an experiment successfully conducted by Castle and Phillips will indicate, even to those who have no special knowledge of the mechanism of heredity, the important role the ovum plays. These investigators removed the ovaries from an albino guinea-pig, and in their place substituted the ovaries of a black guinea-pig. "From numerous experiments it may be emphatically stated that normal albinos mated together produce only albinos." But in this experiment the result was otherwise, for the albino into which the ovaries of a black guinea-pig were grafted produced only black offspring. The color-coat of her young, therefore, was not influenced by her own white hair, but was determined by the eggs really belonging to the black animal from which the ovaries were taken; in no other way can the result be interpreted. It is certain, moreover, that the mode of transmission of material qualities here exemplified is not exceptional; on the contrary there is no doubt that the ovum always conveys the sum total of the qualities the offspring inherits from the mother.
The germinal cells then contain the material basis of inheritance, and in all probability the substance is located within the nucleus of the cells. This substance had been seen and studied long before its relation to the problem of heredity was suspected. Because it takes a deeper stain than the rest of the nucleus, it stands out prominently when the cell is treated with certain dyes, and this property accounts for its name—chromatin. Under such conditions as prevail just before a cell divides, the chromatic substance is broken up and reassembled in the form of rods called chromosomes. Curiously enough the number of rods is uniform for each species of animal, though different numbers are characteristic of different species; the characteristic number for man is twenty-four.
Unless some arrangement was made to prevent it, the act of fertilization would cause the number of chromosomes in the fertilized ovum to be double the number characteristic of the species. In man, for example, the addition of twenty-four chromosomes from the spermatozoon to an ovum that already contained twenty-four chromosomes of its own would mean that after fertilization the ovum contained forty-eight. Such a result is prevented through the process to which we have referred in the preceding chapter as the ripening of the ovum, and also through a similar process in the case of the spermatozoon. These two processes lead to a reduction in the number of chromosomes, so that finally every human germinal cell contains twelve, and therefore when the ovum is fertilized the characteristic number twenty-four is restored. While we know nothing of the forces which determine, on the one hand, what elements shall be discarded by the germinal cells and, on the other hand, what elements shall remain, it is definitely proved that a selective process always takes place. This fact admirably explains the variation in the characteristics inherited by children of the same family. So far as is known, the traits which will be passed on from either parent are a matter of chance. Whatever these hereditary traits happen to be, the best evidence we have indicates that the problem of a child's inheritance is settled once for all the moment conception takes place.
MATERNAL IMPRESSIONS.—Contrary to all that we know of heredity, the conviction prevails among the laity that the character of a child depends greatly upon the mother's surroundings during pregnancy: this is the doctrine of maternal impressions. As is usual with superstitions, this one emphasizes the unfavorable possibilities and holds that the unborn child may be affected by the mother's unhappy thoughts or maimed by her mental distress if she is exposed to unpleasant sights. For this belief there is no foundation; the cases often cited in its support may be fully explained on the grounds of coincidence.
With the possible exception of such individuals as are spending their lives in solitary confinement, there is scarcely a human being who has not in the course of nine consecutive months some untoward physical or mental experience which engraves itself upon the memory. Prospective mothers are not apt to be exempt from a rule so general in its application, but if by good chance one happens so to be she will hardly fail to hear of the misfortune of others, which, according to the doctrine of maternal impressions, may be equally effective in interfering with the proper development of the child. We should then rightly expect most, if not all, babies to be "marked"— clearly a situation which does not prevail.
In order to learn how frequently prospective mothers may have disagreeable experiences which they fear will affect the formation of the child, I have lately asked the patients whom I have attended, "Was there any incident during your pregnancy to which you could have attributed the infant's condition, had it been marked?" The babies of all those to whom the question was submitted were normal; yet without exception those whose pregnancies just completed were their first answered in the affirmative. It is also pertinent that one of these patients had lost her brother by a violent and accidental death when she was four months pregnant; a similar bereavement was suffered by another at the eighth month; each was, however, delivered of a perfectly healthy child. Among those with whom the recently ended pregnancy was not the first I found some who could remember incidents popularly believed to have an influence over the development of the embryo; most of them, however, had given the matter so little thought that they could not definitely recall whether such incidents had occurred or not. From a similar series of observations covering two thousand cases, William Hunter came to the conclusion, nearly two hundred years ago, that there was no support for the belief in maternal impressions.
Whenever a child does happen to develop abnormally, it must be clear that, from the very nature of our existence, some incident can be recalled which will satisfactorily, yet unjustly, bear the blame. It may be confidently said, however, that, for every mother whose fears are realized, hundreds are agreeably disappointed in finding their babies perfectly normal. In the face of so many negative instances it is amazing that any person, even though ignorant of medical teaching, should be inclined to attribute abnormal development to something the mother has seen or heard, thought or dreamt, or otherwise experienced while she was pregnant. Yet unfortunately many do believe this. It is worth while, therefore, to supply further evidence, and thus escape any suspicion of unfairness in argument, to prove that maternal impressions are unable to affect the formation of the embryo.
It is found, as a matter of experience, that the superstition regarding maternal impressions generally begins to cause anxiety during the second half of pregnancy; and then such an influence is entirely out of the question. By the end of the second month the form of the embryo has been definitely determined, and subsequently cannot be altered. It is even true that errors in development are most apt to occur within the two or three weeks that immediately follow conception, and therefore occur at a time when pregnancy is not often clearly recognized. Thus it happens that women begin to worry about the influence their minds will have upon the formation of the child long after its form has been established.
Incidents in the life of a prospective mother are in point of fact equally inert so far as their influence upon development is concerned, no matter whether they occur during the earlier or later part of pregnancy. There is never any anatomical means by which maternal impressions could be conveyed to the embryo. Such an influence would have to be exerted through the placenta; and that is impossible. There are no nerves in the placenta to carry impulses from the mother to the child. Even the blood streams of the two beings are kept apart; and though it is unheard of that the blood should carry nerve impulses, if that happened to be the case, it could not prove effective here, for the blood of the mother does not enter the child. It is nourished by food which passes from the mother's blood, to be sure, but there is no more reason to expect this nutriment to exert an hereditary influence than there is to expect an infant to grow to resemble the cow with the milk of which it is fed. With these two possibilities eliminated, no path can be imagined by which impulses might travel from the mother to the embryo.
Scientific investigation has brought to light these facts, as it has also taught the real causation of the disfigurement once attributed to the mother's mind. Departures from the usual form of the body occur during the earliest days of pregnancy and arise in consequence of some irregularity in the process which molds the body-form from a simple spherical mass of cells. Why irregularities sometimes occur is not altogether clear; except in so far as it has been determined that the fault lies within the embryo itself. Whenever these defects are associated with events which have disturbed the mother's mind, it cannot be other than a simple coincidence.