As regards the other bones of the body, all that need be said here is that they are preceded by the structure which we know as cartilage, and in the bones of the limbs at two or three different points this cartilage begins to be transformed into bone. These points are known as centres of ossification.
CHAPTER XI
HOW THE EMBRYO IS NOURISHED
Having noted how the embryo itself takes its origin, and then studied something of the beginnings of some of its most important parts, we may now very briefly refer to the subject of its own nourishment. This has more than a mere academic interest, because obviously the proper growth and development of all the various tissues and structures in the embryo must depend ultimately upon the nourishment with which they are supplied. Their own inherent characters cause them to divide and subdivide so as to give rise to the millions of cells which are required to make the body, but these cells, in their turn, are dependent upon outside sources for the nourishment which enables them to keep on growing, or to maintain their full growth when they have arrived at that stage.
Nature has made many varied arrangements for this nutrition during embryonic life in different classes of animals. In some a considerable quantity of yolk is so arranged with reference to the embryo that the latter can draw upon it for some time for its supplies. This is the case, of course, in birds, and in some reptiles. We need here, however, only consider the case of the human embryo.
Three sets of structures are concerned in human embryonic nourishment, namely, the Allantois, the Villi of the Chorion, and the Placenta.
The Allantois is developed in the form of a hollow bud from the posterior part of the primitive alimentary canal, and ultimately comes to form the umbilical cord, and the embryonic part of the placenta. It is this structure, the allantois, which allows at a very early date of the embryo establishing a blood-connection with the maternal tissues, and hence it plays a very important part in the transmission of nourishment to the embryo. Not only does it do this, but it allows of the removal of waste products.
The villi of the chorion are outgrowths by means of which the very early embryo attaches itself to the walls of the cavity, which it has made for itself in the wall of the uterus. As they grow larger, these villi push their way into many of the small blood-vessels in the uterine wall, and so come to lie actually in a mass of blood from which they abstract the elements of nutrition. At first the villi themselves contain no blood-vessels. Nourishment passes through them by a simple process of osmosis. Later on, vessels grow into the villi themselves. The nutriment supply is secretion, in the first place, of the uterine glands, which these villi easily absorb. This process takes place during the first two or three months of embryonic life. At the end of this time most of the villi disappear, and the few that remain take part in forming the fœtal or embryonic portion of the placenta.