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

BOOK 1

body is nude, except for scattered fine hairs, or lanugo. In both, the embryonic lanugo is succeeded by a general outgrowth of fully developed hair, but in man this outgrowth is much slighter than in an ape. Man’s naked skin is less developed than the skin of an ape; it is a case of arrested development.

Before we continue the story, and trace the developmental changes that occur after birth, we must take note of the way in which the embryo is housed and nourished.

We have already noted that the developing germ embeds itself in the wall of the uterus

THE SCIENCE OF LIFE

CHAPTER 4

capillary blood-vessels. Now at the boundary between the chorion, which is composed of embryonic tissue, and the wall of the uterus, which is maternal tissue, the embryonic and maternal bloods come into very close contact, without actually mixing. They are separated only by very thin membranes of tissue. In this region, the placenta, there is a continual interchange between the embryonic and maternal bloods; the embryo receives food and oxygen from the maternal stream, and gives up its waste products to it.

Tt will be seen that the embryo is nourished

as soon as it reaches that chamber. At by a circulatory system of its own, and that the placenta acts as lung, intestine, ‘Placenta and kidney combined. This is why

Oviduct

Umbilical Cord

Fig. 65.

The embryo in the uterus.

The blood-vessels of the embryo are shown black ; those of the mother by close shading.

that stage it consists of a hollow vesicle, the chorion, which contains a little knob where the embryo and certain enveloping membranes will develop. As the embryo grows this chorionic vesicle grows too, so that at a later stage (Fig. 65) it bulges out into the cavity of the uterus, being coated with a thin layer of uterine tissue. The embryo grows and develops, remaining attached to the wall of the chorion by a stalk, the umbilical cord, which sprouts out of its belly. Furthermore, the embryo develops a circulatory system very early, and in addition to the heart and vessels in its own body it has arteries and veins running along the umbilical cord to the chorion, where there is an extensive network of

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the heart is the first of our organs to develop and begin working. From first to last, from the first month to the eightieth or ninetieth year, the incessant heart keeps the body going.

§ 4 Rhythm and Birth

At this point it will be best to digress for a moment from our account of an individual life-history and to note something of the way in which the reproductive processes are controlled and regulated.

We have seen that the condition of the sexual organs is constantly changing, particularly in the female. There is a normal rhythm of growth and destruction in the _ uterus, accompanied by changes in the ovary ; moreover the rhythm affects other parts as well, for at the periodic clean-out the pulse-rate quickens and the temperature of the whole body may rise a fraction of a degree. During pregnancy there are profounder alterations, affecting directly or indirectly every part of the body. The rigorous periodic overhaul to which the resting uterus is subjected is suspended and that organ suffers other changes. Its wall has two layers, an outer muscular one and a lining mucous one containing simple glands and numerous blood-vessels. It isin this inner layer ‘that the embryo embeds itself. When this happens the mucous layer thickens and its blood-vessels enlarge enormously, so that it becomes like a sponge filled with maternal blood ; from this copious supply the embryo absorbs the substances it needs. As the embryo grows the uterus becomes more and more