villous processes of the chorion, which have hitherto covered the latter membrane throughout its whole extent, are now limited to that portion of it in contact with the decidua serotina, or that part of the hypertrophied mucous membrane of the uterus into which the villous processes

Human embryo, natural size, three months old. (HAECKEL.)

of the chorion still remaining insinuate themselves. The fusion of the two constitutes the placenta (Pl. 2, Fig. 1). Of the remaining portion of the hypertrophied mucous membrane of the uterus, that part which ultimately grows around the ovum is known as the decidua reflexa, and

that in contact with the wall of the uterus, the decidua

vera.

The villous processes of the chorion, when examined with the microscope, present so characteristic an appearance (Fig. 33) that their presence may be accepted as positive

proof of the existence of the embryo, even if not a trace of the latter be found. The general appearance of a villous process is like that of a sea-weed originating in the chorion by a trunk which divides and subdivides into filamentous branches, swollen here and there, terminating in rounded bulbous extremities, and consisting internally of a finely-granular substance containing nuclei. At first the

1 Dalton, John C.: Human Physiology, seventh edition, Philadelphia, 1882, p. 647.

villous processes of the chorion are without blood vessels, but with the development of the allantois they become vascular through prolongation of the terminal allantoic vessels, which are disposed in loops. The placenta (Pl. 2, Fig. 2) or after-birth, consisting essentially of the interlacement of the blood vessels of the embryo with those of the mother, is a flattened fleshy, vascular, dish-like mass, round or ovoid in shape and with a diameter of from six to eight inches. It is the organ by means of which food and oxygen are conveyed from the blood of the mother to that of the embryo. The process by which this is accomplished is, however, by osmosis, as there is never at any period of gestation an anastomosis of the maternal and fœtal bloodvessels.

Such being the structure and development of the placenta, it is evident that, just in proportion as the fœtal and maternal portions of the placenta become more and more intimately fused together, the greater will be the difficulty experienced and risk run in the expulsion of the fœtus from the uterus. It is for this reason that an abortion committed after the third month of gestation may be attended with such fatal consequences, the hemorrhage resulting from the rupture of the blood vessels being at times very great. Indeed, at a late period of gestation the maternal blood vessels become dilated into great sinuses, veritable blood lakes. On the other hand, if gestation has not advanced beyond the third month, the embryo may, under certain circumstances, be expelled entire from the uterus, very much as a glove is removed from the fingers, without any serious consequences.

Subsequent Changes Undergone by the Human Embryo.— The subsequent changes undergone by the embryo during the last six months of pregnancy are of interest, medico

legally, as enabling the examiner to state the probable age of a foetus obtained from a supposed case of abortion. At the fourth month the foetus is 5 to 6 inches long and weighs about 3 ounces, the umbilical cord measuring about 7 inches in length. At the fifth month the fœtus has attained a length of 6 to 7 inches, weighs from 5 to 7 ounces, and is covered with the vernix caseosa; the hair of the head and body, or lanugo, is quite distinct, and the umbilical cord is about 12 inches long. If abortion occurs at this period of gestation, the membranes are first ruptured, and then the foetus is expelled from the uterus. At the sixth month of pregnancy the fœtus varies in length from 9 to 10 inches, weighs a pound or more, and meconium is usually found in the intestines. At the end of the seventh month the foetus is usually 14 inches long, weighs two and a half to three pounds, the eyes are open, the membrane of the pupil is disappearing, one testicle has descended into the inguinal canal. If the child should be born at this period, the arms and legs will be bent in the position they assumed in the womb, and it will be viable. At the eighth month the fœtus is sixteen inches long, weighs between three and four pounds; the skin has become thicker, and is covered with fine soft hair; one testicle, usually the left, has descended into the scrotum. At the end of the ninth month, or at full term, the fœtus varies in length from eighteen to twenty inches, and weighs on an average seven pounds. The intestines are nearly filled with meconium; the bladder contains urine; both testicles have descended into the scrotum.

As aiding in determining the age of the foetus, it may be mentioned that at full term the umbilical cord is usually inserted about eight to ten lines below the centre of the body; whereas, at an earlier period of gestation, the point