Development Of The Individual
( Originally Published 1923 )
7. Development of the Individual.—As stated on page 73, the individual begins life as a single cell. In the case of human beings and other higher animals and plants this is a fertilized egg-cell. Immediately after fertilization this cell divides into two, each of these divides again, and so on, the number of cells doubling until several hundred are formed. These cells remain clinging to each other instead of separating, as they do among the lowest plants and animals.
The microscopic mass continues to grow and to change its shape, portions folding in and becoming completely enclosed. At other points portions bud out and become the arms and legs. The head is the first part to acquire distinctness, being much larger in proportion to the rest of the body than it is later in life.
A comparison of the embryos of many different animals, especially backboned animals, shows striking similarities during the first stages of development. Indeed, the embryos of various orders could be distinguished only with great difficulty, or not at all, except by those who have made a special study of them. At one point the embryo of a human being is very much like the embryo of a bird or a rabbit or a fish. Later on it still resembles the rabbit or the bird, but the fish embryo has now become more fish-like. Still later the human embryo resembles the embryo of rabbits or other mammals, but the embryo of the bird has already become distinctly bird-like. And so on, the human embryo becoming distinctly human only after a considerable advance. This does not mean, of course, that at any point in its development the human embryo is a fish or a bird. It means only that all of the backboned animals have many of the early stages in common. Thus, while it is true, for example, that the human embryo has at one point in its development a series of wrinkles on the side of the neck corresponding to the gill slits of fish embryos, it is not necessary to think of the embryo as being at this time a fish.
In birds and lower vertebrates the cells which make up the developing embryo live and grow upon the reserve of food "stored up" in the yolk of the egg. In man, as in the other mammals, the part corresponding to the yolk sac broadens out and finally enwraps the whole embryo. From the surface of this envelope tiny finger-like processes emerge which grow into the lining of the womb, and thus establish a connection between the mother and the embryo. This is the beginning of the placenta. Here the blood vessels of the embryo and the blood vessels of the mother come into close relations, although the blood streams are entirely distinct. Food material diffuses through the delicate membranes, and the blood circulation of the embryo distributes it to all parts of the growing mass.
In about five or six weeks after fertilization the embryo has developed so far as to be distinguishable in its main features as a human being, although it is only about half an inch long. In about two months, when about an inch in length, the embryo has a complete heart, the lungs are formed and their blood vessels are connected with the circulation, the abdominal wall is completely enclosed, and the external genital organs have begun to form, although the sexes cannot at this stage be distinguished.
The blood vessels connecting the placenta with the body of the embryo lie within the "umbilical cord." At the time of birth the cord is broken or cut close to the abdomen, and the shriveled stump becomes the navel or "belly but-ton." Then the placenta detaches itself from the wall of the womb and comes out as a part of the "after-birth."
The changes that are taking place in the mother during pregnancy seem to depend, at least in part, upon the interchange of materials through the embryo's placenta, or perhaps upon chemical changes taking place within the placenta itself. The development of the mammary glands is the most striking of these changes, although the growth of the womb and modifications related to the later delivery of the baby are also of great importance. The dependence of the new individual upon the mother for food, warmth, oxygen, and the removal of wastes, represents a substantial drain upon the mother, and it is in a sense a first lien upon her, since very often an undernourished or overworked mother will give birth to a perfectly healthy and well-developed infant. In what-ever manner we may account to ourselves for the very evident sacrifice of the parent for the child, it is apparent that the new protoplasm represented by the infant, in its parasitism upon the parent displays more vital energy than the parent, by taking to itself from the common supply all that it can use, even to the exhaustion of the mother.
Even under the most extreme conditions of disparity, however, the mother is generally very glad to do all she can for the young, and is not aware of making a sacrifice. As a rule it is only where there are special social or economic difficulties in the way that the mother fails to develop a great deal of affection for the unborn child; and this affection of the parent for the child is in turn the highly specialized and well-defined affection which the infant begins to develop in a vague way from the very first. Men and women who have had happy child-hoods are more likely to care for children and to make way for them.