The Embryo : Part 5

The Prospective Mother: A Handbook for Women During Pregnancy
by J. Morris Slemons

(Page 8 of 18)

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.