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

BOOK 1

dividing, the cells have begun to arrange themselves in a definite pattern. They build themselves into a hollow ball, with a thickened knob projecting into its cavity at one point. At this stage the developing germ eats its way into the wall of the uterus, actually destroying some of the maternal cells, and becomes completely embedded. It lies in a little burrow, surrounded on all sides by uterine tissue and bathed by the blood of the mother. As a result of its implantation the germ has become a parasite upon its mother and is in a position to absorb nourishment. In the oviduct it had no external food-supply and was living on its own store of yolk, so that it was unable to

Brain

Gill Clefts

Tail

63. Human embryo, about three weeks old. Ten times larger than life.

Fig.

increase in weight; but now it is bathed by blood charged with food substances, so it can absorb what it wants and build up new embryonic tissue. Hitherto it has been simply developing ; now it develops and grows at the same time.

For a few days the tiny vesicle differentiates and elaborates itself, but without showing any trace of the organs of the future baby. The first well-defined structures to appear are not parts of the baby at all; they are the apparatus of membranes and ducts that is to wrap round the embryo and protect and feed it. Towards the end of the second week of human development the germ is little more than a millimetre in diameter, and consists of three hollow

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THE SCIENCE OF LIFE

CHAPTER 4

vesicles, two lying inside the third ; moreover the cells forming the walls of these spheres have become arranged into sheets or layers. During the third week the whole apparatus grows rapidly, and at this time the first visible organs of the embryonic body appear. On the wall that separates the two inner vesicles there is a thickened disc, about a millimetre and a half across; traversing this disc there is a faint furrow, the primitive streak ; and just in front of this furrow two thickened ridges are beginning to form, ridges that will close together and give rise to the brain.

The tiny disc grows and differentiates very rapidly. By the end of the fifth week of development all the important organsystems are already laid down. The embryo is now about one-fifth of an inch in length and possesses a beating heart, a definite nervous system, little sprouting buds that will become limbs, relatively large eyes, a rudimentary kidney, a growing digestive tube, and even a well-defined mass of cells that will eventually give rise to gametes, and replace the embryo by new individuals when it has developed and lived and is in its turn worn out. The essential systems are already present. But in its anatomy, in the way these parts are fitted together, the embryo presents at this stage some curious features that are destined to disappear as growth proceeds. We will take careful note of these points, for they are of the first importance in the understanding of Comparative Anatomy and of Evolution.

One of the most obvious parts of the human embryo of about the fourth week is a prominent tail. This tail is clearly shown in Fig. 63. It curls forwards and upwards and has a muscular development which rivals in massiveness that of the trunk. That is an important point. The tail of an early human embryo is not like the slender termination of an adult monkey, or mouse ; it is more like the broad powerful tail of a fish. In the normal course of development the growth of this tail is relatively slow, and the rest of the body grows over it and encloses it, so that by birth the vertebral column ends in a little forwardly curving hook, the coccyx, which is completely embedded in the lower end of the trunk. But sometimes development is not quite normal. Not uncommonly the medical student, dissecting a human body, finds vestigial and perfectly useless muscles passing to the coccyx. And very occasionally a child is born into the world with a tail—an appendage which is little better than a soft piece