After the third month the embryo is nourished by the placenta itself, which is at this stage developed. As we have seen, it arises partly from the villi of the chorion, which is its embryonic portion. The other part of it is maternal in origin, arising from the portion of the uterine wall which is immediately over the embryo. The connection between this structure, the placenta and the embryo, is constituted by means of the umbilical cord. The function of the placenta is partly to supply nutrition, partly to serve as an organ of respiration for the embryo, whose lungs are, of course, not functional, and partly it acts in the same way as the kidney does in after life, by excreting certain products. From the placenta the embryo derives those food elements at first provided by the secretion of the uterine glands. Afterwards these elements are supplied by cells which lie between the fœtal villi and the blood of the mother. Its respiratory function consists in allowing oxygen and carbonic acid gas to pass by osmosis between the embryonic and the maternal blood. The process is exactly analogous to that which takes place between the gills of a fish and the water in which the fish lies. Of course, it will be easily understood that there is as yet no great need for a large supply of oxygen, because the embryo is merely growing, and not using its various organs.
It should be clearly understood that under ordinary conditions of embryonic life there is no direct mixture of the blood of the mother and that of the developing embryo. All the processes which contribute to its growth and maintenance, including those of respiration and excretion, take place through the intermediate structures above mentioned. This is an extremely important point, because it means—and evidently that is the object of the arrangement—that there may be much of an injurious character in the blood of the mother which never reaches the embryonic tissues at all. Doubtless the cells which form the organs of nutrition for the embryo have a capacity for selecting the elements required for purposes of nutrition. It is their business to look after this process. How perfectly it is performed can at once be understood when we recollect how very frequently the tissues of the mother herself are in anything but perfect health, and yet the embryo is born healthy. Were it not for this intermediary process, the embryo could hardly help being poisoned or otherwise injured by all the varied unhealthy products and substances which the ignorance of some mothers allows to be present in their blood during this important period. Even with this means of protection, the maternal blood may be so utterly deficient in nutritive qualities, or so actively injurious from saturation with alcohol, or from some equally toxic substance, that the fluids which reach the embryonic cells may be very much impaired in quality. Nevertheless, it is astonishing how much danger can be avoided in this way by Nature's provision in the method of nourishing the embryo.
If the development and growth of the embryo in a human being runs a perfectly normal and uninterrupted course, the following points could be observed at various stages. At the end of the fourth week in growth, the embryo is distinctly curved, so that the two ends—the head and the tail—are close together, the whole being about half an inch in length. Even at this very early stage, the canal which gives rise to the brain and spinal cord is closed in. The vesicles of the eye and the ear have both made their appearance, and the limbs are just beginning to show as buds. The heart is quite obvious, and its division into its four chambers is commencing. In another four weeks the embryo has reached the size of one inch, and the head is beginning to take on a shape more resembling that associated with a human being. The tail, on the other hand, has now disappeared. The limbs have grown to the extent that both hands and feet are starting growth, and in the region of the head both the eyes and the ears, as well as the nose, can be distinguished. Even at this stage, however, the sex of the embryo cannot be made out. A month later, at the end of the twelfth week, a considerable development has taken place. The embryo is now about three and a half inches long. There is a general growth to be observed, and the bones are beginning to ossify. In sixteen weeks, when the embryo measures about five inches in length, the sex is easily distinguishable. The most characteristic thing for the weeks succeeding this is the relatively large size of the head, upon which hair appears at about the twenty-fourth week. In twenty-eight weeks the embryo should weigh about 2½ lb., that is to say at the seventh month of embryonic life. Should the child be born at this time as the result of any of the causes which give rise to premature birth, there is a possibility that it may live, though as a rule it does not. Four weeks later it should weigh 3½ lb., and if born now may frequently live, if carefully attended to. In another four weeks the embryo is nearly eighteen inches long, and weighs about 5½ lb., and the body has a more rounded appearance, because by this time there has been a considerable growth in fat. If born at this stage it ought to be quite possible to save the life. Finally, at the end of forty weeks, the normal full embryonic period of human life, the healthy child should weigh about 7 lb., having smooth, pink skin, and being otherwise perfectly developed.
CHAPTER XII
RECAPITULATION
In bringing our study of Embryology to a close, we may glance briefly at another aspect of the subject, namely, that which emphasizes the fact that in its development the embryo recapitulates the history of its ancestors.
It is quite obvious that the offspring of any species of animal, if they are to live and survive in the same kind of environment as that in which their parents live, must resemble them somewhat closely. The only way in which Nature can secure such a sufficiently close resemblance of offspring to parents is by insuring that they should develop along similar lines. So it is that we find that the whole of the life history of an individual is more or less a recapitulation, with, of course, variations, of that of the parents and ancestors. Each successive step from the very beginning of the fertilisation of the ovum repeats a stage through which previous generations have passed. If from any accident a step in this recapitulation is omitted, the embryo is to that extent deprived of some feature possessed by a parent or ancestor; and if this be a sufficiently important omission, it is impossible for such an embryo to survive. That is one way in which an embryo may differ from its parents. That is a retrogressive change. On the other hand, such an embryo, in addition to recapitulating the stages through which its parents passed in development, may have something new added, something which appears for the first time. In other words a progressive variation may appear.
Now, since the embryo follows the same developmental track as did the parent, passing through the successive stages of germ-cells, fertilised ovum, embryo, fœtus, infant, child, youth, and adult, it follows that should it exhibit any additional peculiarity, unnoted in the parent, the embryo has obviously varied progressively. That is to say, it has pursued the same line of development together with some new addition. On the other hand, should the offspring at any of these stages in its career be obviously without some of the characters of previous generations, it is as certainly due to the fact that the recapitulation of the history of development has been, in that particular, incomplete.