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.