In mammals generally the embryo is distinctly in process of formation before the double membranes grow up and envelop it (p. 95); in man and anthropoids the formation of the embryo remains in abeyance until the membranes are developed. Once these are completed, the small knob of cells which represents the embryo begins to take on a definite shape. The reader will have observed that I have often employed posture to explain the many points of similarity in the structure of anthropoid and man, and will at last feel certain that in this case no such explanation can be offered. On the contrary, it is my opinion that, by some mechanism which we do not know at present, the early development of the embryo has been changed by the posture of its parent. The uterus is situated in the lowest part of the abdomen. In upright primates it is exposed to a special degree of pressure, not only from the weight of the superincumbent viscera, but also from the compressing action of the musculature of the abdominal wall which comes into action in balancing and in all great muscular exertion. Whether this explanation will stand the test of time or not, the fact remains that the resemblance which we see between man and the anthropoids in the adult stage is already present in the very earliest stages of development.
It will be now apparent to the reader why the various stages in the ancestry of man are so dimly represented during the development of the human body. It is because all the processes of development are modified to adapt the embryo and fcetus to an intrauterine and parasitic life. We cannot, however, explain the origin of the human body from a single cell, the appearance of gill arches, the formation of a cloaca and the occurrence of other developmental processes, unless we suppose man to have been evolved from the very lowest forms of animal life. We see traces of various stages of his evolution. Many of his most primitive embryological structures he shares with other vertebrates. These have not been mentioned. For example, we see during his development three sets of renal organs appear, the one succeeding the other. There is first the pronephros or head kidney which persists as a small appendage of the testicle of man and of the Fallopian tube (oviduct) of woman. Then a second kidney—the mesonephros or Wolffian body appears. This renal organ forms part of the seminiferous duct system in man and a vestigial organ in the broad ligament of the uterus in women. It is not unfrequently the seat of disease in women. Then the third or final renal organ—the kidney—appears. In having a triple succession of renal organs man is like other mammals.