When the embryo has finished its journey, and has settled itself for a long stay of nine months, not of rest, but of ceaseless activity, of growth and development so marvelous and sure, it begins to draw its life from the uterus, for the stored up food in the ovum is already exhausted. At first it draws by absorption through the membrane enclosing it, then through the placenta or “afterbirth,” which is created as the medium of communication for the life-giving force, between mother and child.
Up to the close of the third month we call this little new life an embryo; after this time it is called a fœtus. For a full description of the embryo and fœtus, in the various stages of development, we copy from Leavitt’s Science and Art of Obstetrics.
“The First Month.—The embryo in the first week of gestation, is a minute, gelatinous and semi-transparent mass, of a greyish color, presenting to the unaided eye no definite traces of either head or extremities. The entire ovum measures but one-fourth of an inch, and the embryo but one-twelfth; but during the next week they double in dimensions. The coverings of the child are developed, and it is attached to the uterus, but does not yet draw its life from it. At the close of the month the ovum is about the size of a pigeon’s egg, and weighs about forty grains. The embryo is about three-fourths of an inch in extreme length, and about one-third of an inch in direct measurement as it is coiled up. The structures have so little bulk, that when ruptured they easily escape attention, in abortions, generally passing with a clot.
“Second Month.—At eight weeks the ovum is about the size of a hen’s egg, and it weighs from one hundred and eighty to three hundred grains, and is about two-thirds of an inch in length from head to caudal curve. Its independent circulatory system is forming: indications of the external generative organs are visible: and ossification has begun in several parts of the body.
“Third Month.—The embryo weighs from three hundred to four hundred grains, and measures from two and a half to three and a half inches in length. The forearm is well formed and the fingers are discernible. The umbilical or navel cord is about two and a half inches in length. The head is relatively large, the neck separates it from the trunk, and the eyes are prominent. The chorion has lost most of its villi, and the placenta is formed. Points of ossification are present in most of the bones. Thin membranous nails appear on the fingers and toes. Sex may be determined by presence or absence of the uterus.
“Fourth Month.—The fœtus weighs five or six ounces, and is about five inches long. Its sex is more distinct; movements are visible. The convolutions of the brain are beginning to form: ossification is extending: the placenta is increasing in size, and the cord is about twelve inches long. The head is one-fourth the length of the whole body. The sutures and fontanelles are widely separated. Hair begins to appear on the scalp. If born, the fœtus may live three or four hours.
“Fifth Month.—Fœtal weight has increased to ten ounces, and length to about nine inches. The head is still relatively large. Fine hair, (lanugo) appears over the whole body. Fœtal movements can be felt by the mother. If born the fœtus can live but a few hours.
“Sixth Month.—Weight about twenty-four ounces, length eleven inches. Fat is found in the subcutaneous cellular tissue. Hair is darker and more abundant. The membrana pupillaris exists but the eyelids separate. If born at this time the fœtus breathes freely, but life is retained only a few hours, with rare exceptions.
“Seventh Month.—Weight from three to four pounds, length fourteen to fifteen inches. The skin is wrinkled, of red color and covered with vernix caseosa. The pupillary membrane disappears. If younger than twenty-eight weeks it is not likely to live.
“Eighth Month.—Weight from four to five pounds, length from sixteen to eighteen inches. Development is now rather in thickness than length. The nails are nearly perfect, and the lanugo is disappearing from the face. The navel is gradually approaching the centre of the body, until now it has nearly reached the median point. The cranial bones are easily molded under pressure, a point to be remembered as bearing on the question of induced labor in pelvic deformity.