The reader will observe that I am not following the orderly methods of those who write text-books of anatomy ; there is no attempt here to describe one part of the body after another in regular sequence. My aim is rather to bring forward the general opinions held by those who have made a special study of the human body. In the four chapters which precede this one I have touched on the evidence which has led us to regard man as having arisen in a long past period in common with the animals to which he is allied in structure. If such is his origin, then we ought to find further evidence of it during the various stages he passes through before birth. Indeed, it was at one time expected that when we came to know all the changes which the human body passes through from conception to birth, we should have a complete picture of the past history of mankind. It was expected that the embryo would recapitulate the features of its ancestors from the lowest to the highest forms in the animal kingdom. Now that the appearances of the embryo at all ages are known, the general feeling is one of disappointment; the human embryo at no stage is anthropoid in its appearance. The truth is, we expected too much ; we failed to realize that higher animals are adapted for two lives—one a peculiar vegetative existence within the womb, and the second a conscious life which commences with birth. The most marvellous adaptations to be seen in our bodies are those which relate to the phase before birth. At a very early stage of development we see the embryo become enveloped within a double-coated hull or membrane, developed as an outgrowth from its own body. Within these membranes the embryo develops, but there never was, there could not be, an individual or ancestor which passed its whole life thus enclosed within an envelope of foetal membranes. The inner of these membranes—the amnion—is filled with clear fluid within which the embryo floats, evenly supported on all sides and protected from irregular pressures which would distort its delicate growing tissues. The outer membrane—the chorion—gives rise to the placenta which fixes itself to the mother's womb and draws from it nourishment and oxygen for the supply of the embryo, which as is grows and takes on a definite form comes to have the name of foetus applied to it. The embryo develops its own blood and its own vessels ; its circulation at all times is distinct from that of the mother; there is no intermingling of maternal and fcetal blood. The foetus is a parasite ; beginnings of a digestive and of a respiratory system appear in it, but they do no work; these functions are performed by the placenta. All the early stages of development of the embryo are marked by the efforts to produce the structures which are necessary for fcetal life. The provision of such structures marks the true nature of the embryo. The conditions under which the embryo and foetus live, grow, and draw their nourishment, are very different from those which surround low forms of animal life. We cannot expect the embryo to reproduce for us the early ancestral stages of life ; these have been much modified and some of them replaced to suit the phase of existence within the womb.
Yet in a broad way we see various stages indicated during the building up of our bodies. The fertilized egg or ovum, which forms the commencing stage for each one of us, represents in its structure the lowest forms of animal life —the protozoa. The ovum, like a protozoon, is a cell with a nucleus—so small that seven hundred of them could be laid side by side on a line scarcely an inch in length. The ovum divides, each cell thus formed redivides, the cells arranging themselves so as to form a minute plate-like structure—the embryo. It is impossible to explain certain appearances of the embryo, the streak seen along it, and an opening—the blastopore or primitive mouth—which perforates the embryonic plate, unless we suppose that we had in our ancestry a form built on the same type as the hydra made familiar to us in our text-books on biology. The blastopore is regarded as representing the mouth of a hydra-like animal; it is soon closed in the human embryo, and by the third week there is a new mouth formed at the opposite end of the embryonic body, which becomes the permanent one. The early formation of the fcetal membranes, which enclose the embryo, and the accumulation of yolk within the embryo—an early provision for nourishment—mask and disturb the hydra-like appearance of the embryo. In the next stage we see the rudiments of the body cavity being formed—the pleural and peritoneal cavities which enclose the viscera of the thorax and abdomen. During the development of worms and many other invertebrates, we see body cavities being produced in essentially the same manner as in us, and we therefore suppose that our ancestry and theirs must have passed through a common stage. At the end of the second week we see transverse lines appear one after another on the upper or dorsal surface of the embryonic plate, which mark out the embryonic body into a series of segments, very similar in appearance. These body G segments remain particularly distinct in worms, insects and crustaceans ; they can also be seen in all vertebrate animals, when the skin is removed. In fishes, the segmental arrangement of the muscles, as well as of the vertebrae and ribs, is especially distinct. In our bodies the vertebrae and ribs still mark the primitive segments, and the intercostal muscles and some of those which act on the spine still preserve the original segmented condition. By the end of the third week, when the human embryo is about one-sixth of an inch in length, the process of segmentation is complete. In a broad way, in spite of special developmental adaptations, the human embryo does recapitulate some early stages of evolution.
It is during the third week that there appears on the neck of the embryo one of the most remarkable manifestations of a past stage of existence. On each side four grooves or depressions are formed. There can be no doubt that these represent the gill slits of fishes. We have every reason to suppose that the arches on the front and sides of the neck of the embryo represent the arches which carry gills, for into each one of them the aorta from the heart sends a branch, in the same manner as may be seen in the gill arches of fishes. No gills are actually developed because the placenta serves their purpose—that of respiration. Indeed, the clefts never open, but part of the first cleft remains, and forms the ear passage. Our ears are developed round the upper part of the first cleft. The lower jaw is developed in the first arch ; the hyoid bone, on which the base of the tongue is set, is formed within the second and third arches. The cartilages of the larynx—the thyroid and cricoid, and also the cartilaginous rings of the trachea and bronchi, are formed within the fourth and fifth arches. It is passing strange that we can recognize the same skeletal parts in our throats as we can see in the gill region of fishes. By the sixth week all outward appearance of gill slits is lost; the manner in which the hinder ones disappear is instructive. In certain fishes the gills are covered by a lid or operculum, which grows back over them from the second arch, thus enclosing a gill chamber. In the human embryo a similar process is seen to occur. The third and fourth clefts are covered by a fold which grows backwards over them from the substances of the second arch, and as it grows back the clefts and gill chamber are obliterated. This is not always the case, however. The surgeon is frequently consulted by patients who complain of a discharge which escapes from a small opening situated on the one or the other side of the neck just above the inner end of the collar-bone. When a probe is passed within the opening it passes upwards in the direction of the larynx for perhaps an inch or more. Such a fistula or opening represents the unclosed gill chamber. Occasionally other remains of gill clefts are found as tags of skin or as auricular appendages on the upper part of the side or front of the neck. In some breeds of goats such appendages occur constantly.