There are two arteries in the cord; these arise from the abdominal aorta in the fœtus; they go by a flexed and tortuous course to the placenta, where they ramify and are distributed. There is only one vein which returns the blood from the placenta; there the radicles coalesce to form the branches; these unite to form the umbilical vein. This is not as flexuous as the arteries, which, being longer, wind around the one venous trunk. After the third month these may be plainly seen in the sheath imbedded in what has been called _Wharton’s gellatine_. Ordinarily the cord lies free and loose in the cavity of the amnion, but occasionally owing to the movements of the child it may be coiled around the child’s neck, be tied in knots, or it may escape below the head so as to prolapse during labor. The length of the cord varies; it is very rarely less than eight inches, and it is sometimes six or eight feet long. After the birth of the child, the pulsation in the cord ceases within about fifteen minutes. After the cord is cut that portion that is attached to the umbilicus dies and usually falls off about the fifth day. The blood of the fœtus is ærated or undergoes a change in the placenta analagous to the change that our blood undergoes in the lungs. CHAPTER VIII. THE FŒTUS. The embryo first begins to be distinct about the _third_ week; is then about two lines long, weighing one to two grains; is surrounded by an amnion which lies loosely about it, and obviously proceeds from the abdominal laminæ; it presents cerebral vesicles; there is the appearance of an eye, several arteries are seen though not distinctly formed; the abdominal cavity is open for a considerable extent in front. About the _fifth_ week the embryo becomes more consistent; the head is disproportionately large; rudimentary eyes are indicated by two black spots; the abdomen is nearly closed, though at the umbilical aperture a loop of intestine escapes; the abdominal members are present, and the cord exists in a rudimentary condition; the embryo is nearly two-thirds of an inch long and weighs about fifteen grains. The successive changes in the development were, 1st, a germ membrane visible immediately after the bursting of the vesicle; 2d, at some point an aggregation of granules forming the cumulus of the blastoderma; 3d, the embryo developed lying at this point, as it were upon the membranes, which consist of three superimposed laminæ or layers; 4th, on the serous layer arise the organs of animal life, the brain and spinal marrow, organs of sense, skin, muscles, tendons, ligaments, cartilage, and bone; on the mucous the organs of vegetative life, the intestinal canal, lungs, liver, spleen, pancreas and other glands. The heart and vascular system arise from the vascular layer (if this can be considered a separate one).