As the attachment between the sac which encloses the egg more firmly and securely adheres to the walls of the womb the filaments which had formed upon all sides of the enclosing sac begin to disappear, except upon the side in contact with the womb. Upon that side they become more pronounced, and in the third month the permanent attachment known as the placenta, through which the embryo is to receive its increasing supply of nourishment from the body of the mother, begins to be formed. The placenta is circular in shape, from one to two inches in thickness in its thickest part, and about six or eight inches in diameter. It forms the temporary medium of communication between the life of the mother and that of the child, and, properly speaking, is not a part of either. It is formed for a temporary use, which terminates with the birth of the child; and at that time the placenta, the umbilical or navel cord, and the membrane we have named, constitute what is called the afterbirth.
The placenta is a flat, soft disk which is attached to the walls of the womb connecting the embryo by the navel cord, through which it respires or breathes, and receives nourishment, and discharges the worn-out particles of matter. It is through this attachment or cord that an intimate connection is established between the growing life of the ovum and the currents of life which flow in the mother's body. Through this placentic cord the embryo receives oxygen and all the elements necessary for its growth, upon the one hand, while, upon the other, it also transfers to the body of the mother the carbonic acid gas and other impurities which in the process of life are necessary to be thrown off.
CHAPTER XV.
THE CHANGES WHICH PRECEDE, ATTEND AND FOLLOW CONCEPTION AND CHILDBIRTH.
Continued.
But while these changes have been taking place within the womb itself, and the decidua or thickened membrane has been forming about this developing germ of life, let us consider some of the changes which have taken place within the ovum itself. If observant when opening a hen's egg, the observer will notice that the yolk is covered by a very thin membrane which encloses and separates it from the other portion of the egg, and holds it in its rounded form. The membrane is exceedingly delicate, but sufficiently thick and strong to serve its intended purpose. Now, the human ovum or egg consists of a vitellus or yolk, which is covered by a similar membrane, known as the vitelline. When this ovum or egg has been fertilized or impregnated, remarkable changes take place. This vitellus or yolk undergoes a series of segmentation or dividings which are known as spontaneous segmentation. The single minute yolk divides itself into two smaller balls or segments. These again subdivide into four; these four subdivide into eight, the eight into sixteen, and so on, resulting in the rearrangement of the yolk into a finely divided granular mass. While this division is in process, the divided parts arrange themselves orderly about the inner surface of the vitelline or yolk membrane, with the minute microscopic spaces between filled with a transparent fluid. While these cells have been multiplying and become so abundant as to be flattened against the internal surface of the yolk membrane, they have developed into true animal cells. The edges of these cells, where they come in contact with each other, form a continuous organized membrane, which lies just within the yolk membrane. This membrane is called the blastodermic membrane.
This new membrane, formed within the sac which originally enclosed the yolk of the ovum, now divides or separates itself into two distinct layers, known as the outer and inner blastodermic membranes, also called epiblast and hypoblast. The egg at this stage of development presents the appearance of a small round sac, the walls of which consist of three layers, each succeeding layer lying immediately within and in contact with the other which encloses it. The outer one of the three is the primitive yolk sac, the second is the outer layer of the blastodermic membrane, and the third the inner layer, while the interior cavity of the egg is filled by the transparent fluid previously mentioned.
In order to understand the beginnings of life it is important to have a clear conception of these different sacs, for these two membranes lying within the yolk sac, and together known as the blastodermic membrane, and separately as the outer and inner layers of the blastodermic membrane, contain the anatomical elements from which the organized being with its fully-formed body is to grow. Indeed, it may be said that these two blastodermic membranes are the body of the embryo which is to develop into the fully-formed physical man.
The noting of this division into two separate layers is important, for the outer one develops into the skin, spinal cord, muscles and bone, while from the inner layer is formed the intestinal canal and the organs of vegetative life.