The science of life : fully illustrated in tone and line and including many diagrams

WEARING OUT OF MACHINE AND ITS REPRODUCTION

of string, but which nevertheless contains the tapering end of the vertebral column. On looking at the throat of an early embryo we find another curious point. At the sides of the throat there are four pairs of clefts—clefts which quite obviously correspond to the gill-slits of fishes (Fig. 63). This correspondence may be traced, not only in the position of the clefts but also in their skeletal supports—the gill-arches—and in the blood-vessels of that region. As development proceeds this early arrangement is modified beyond recognition. The gillarches are converted into the ear-ossicles, into the hyoid bone (a bone lying in the throat, to which the muscles of the tongue are inserted), and into the cartilages of the larynx ; the arteries are reduced in number, only a few of them persisting as part of the great vessels in the thorax; the clefts are filled up and disappear, except for the first, which gives rise to the Eustachian tube and middle ear. takable ; instead of developing a human throat simply and directly, each of us during our embryonic life lays down a gill-apparatus like that of a fish, and then modifies it and twists it about until it becomes a human throat. It should be remembered that as far as the embryo is concerned the gillapparatus is never used for breathing, and indeed never develops the actual respiratory filaments that are found in the gills of fish. Further, on dissecting such an embryo, we should find other conditions that resemble those seen in the lower vertebrates. In the arrangement of its principal veins the embryo is distinctly fish-like. It has a simple heart with only one auricle and one ventricle like the heart of a fish, and in the fact that the urinary, genital, and rectal passages all open into a common pouch, the cloaca, which in its turn opens to the exterior, it differs from adult mammals. These points, again, are altered and corrected as development proceeds. Finally, the early embryo has a skeletal structure running down the backthe notochord—which is destined to disappear and be replaced by vertebrae. We shall deal with the interpretation of these facts clsewhere ; for the present it is enough to note this fish-like stage in our development. During the second month of development the embryo grows and differentiates further and straightens out this fishiness through which it has ascended. By the end of this period it is already recognizably human. An eight weeks’ embryo is about an inch long; it has already got fingers and toes, and its tail is reduced to a hardly noticeable pimple.

Nevertheless the fact is unmis-.

There is still a long way to go—the cerebral hemispheres, for example, are very poorly developed, and the face is grotesque and almost horrible—but the general form is laid down. The embryo is beginning to look like a baby mammal.

In the first two months, then, all the most drastic developmental changes have occurred. The tissues have been differentiated and the general form has been assumed. In the remaining seven months the embryo undergoes various minor adjustments ; its limbs, for example, grow relatively rapidly and acquire more reasonable proportions, and its face becomes human. But the most important process that occurs in this long period is simple growth—the expansion of an embryo barely an inch long into a new-born child.

Human embryo, about four weeks old

Fig. 64. (six times larger than life).

It is curious to compare the late human embryo with that of an ape—a chimpanzee, say, or a gorilla. In either case the seven months’ embryo has the soles of its feet turned towards each other like the palms of its hands. In the ape this primitive condition is retained ; indeed, the foot develops very like a hand, and is used for grasping things. In ourselves the foot becomes modified to support our weight in the erect position; the legs are straightened; the sole is planted flat on the ground, and the bones become stronger and less mobile. The foot becomes an efficient support at the cost of freedom of movement. On the other hand there is another respect in which we, and not the apes, retain an embryonic character. The seven months’ embryo of an ape or a man has thick hair only on the scalp, eyebrows, and lips; the rest of the