It begins to be formed about the end of the second month of gestation, but is not fully developed until the end of the third month. At full time birth its long diameter is six to eight inches, while its greatest thickness is from two thirds to one inch; its weight is about twenty ounces, and, roughly speaking, it is about the size of a soup plate.
It is partly foetal and partly maternal in origin, and exceedingly vascular. The maternal and foetal bood vessels come into the closest possible relationship to each other, only the thinnest membranous septum separating them.
But the maternal and foetal bloods never mix, there being no direct communication between the two circulations, and yet by diffusion, or osmosis, there is an interchange of nutritive elements and gases, constituting nutrition and the equivalent of respiration.
The umbilical cord, or navel string, connects the foetus and placenta; it contains two arteries and one vein; through it the foetus derives its nourishment from the placenta, and also gets rid of its waste products.
The placenta, navel string, and foetal and maternal membranes together constitute what is called the "after-birth", or "secundines", which are "born" usually about twenty minutes after the birth of the child.
The Growth and Development of the Human Foetus. The ovum having been impregnated, the phenomenon of segmentation follows until a morula is formed. Then, probably on the thirteenth or fourteenth day, there is the appearance of the medullary groove and cephalic expansion, which give the earliest indications of the embryonal form.
The neural or spinal canal having been formed, there develops in it a rod of nerve tissue, the anterior extremity of which enlarges to form the brain.
Thus the nervous system is among the first of the structures of the body to be formed.
By the end of the second week the primitive heart appears in the form of a tubular cavity, when the embryo is only one eighteenth of an inch in length.
At the end of the second week, or beginning of the third week, the heart is actively beating, and at the end of a month the four chambers of the heart have formed.
The brain vesicles can now be seen, and the rudiments of the eyes and ears are differentiated.
As early as the twenty first day the limbs begin to appear, as well as the elements of the eyes, nose and mouth.
During the fourth week the growth of the embryo is more active in regard to its changes of form and feature than at any other time.
It now changes its attitude, so that from being erect it becomes strongly flexed until the cephalic and caudal extremities meet or even actually overlap.
At the end of the fourth week the whole "ovum" is about the size of a pigeon's egg, the heart has increased in size and power, the rudiments of the limbs are prominent, the primitive intestine is well formed, and the vertebrae and nerve centres are distinct.
In the second month the eyes are distinctly seen, the external ear has appeared, and the kidneys are formed. As early as the fifth or sixth week the nose and mouth are formed, and the fingers and toes can be seen. In the second month, also, the external sexual organs are formed, though it is not yet possible to determine the sex, for male and female are apparently identical in their early development. At the end of this month the ovum is about the size of a hen's egg, and the contained embryo from one inch to one and a half inches in length.