obliterated, and indistinguishable from the adjacent parts; the heart loses the form of a single heart; a crescentic fold begins to mark the future division into two ventricles, and gradually extends until the septum between them is completed. It is also continued along the bulb of the aorta, dividing it into two trunks, the aorta proper and pulmonary artery; at the upper part the division is left incomplete, so that there is an opening from one vessel to the other, which forms the ductus arteriosus.[27] A similar process takes place in the auricles, the foramen ovale being apparently formed in the same manner as the ductus arteriosus; these changes commence in the human embryo about the fourth week, and are completed about the seventh. At first the body of the embryo has a more elongated form than afterwards, and the part which is first developed is the trunk, at the upper extremity of which a small prominence less thick than the middle part, and separated from the rest of the body by an indentation, distinguishes the head. There are as yet no traces whatever of extremities, or of any other prominent parts; it is straight, or nearly so, the posterior surface slightly convex, the anterior slightly concave, and rests with its inferior extremity directly upon the membranes, or by means of an extremely short umbilical cord. The head now increases considerably in proportion to the rest of the body, so much so, that at the beginning of the second month, it equals nearly half the size of the whole body: previous to, and after this period, it is usually smaller. The body of the embryo becomes considerably curved, both at its upper as well as its lower extremity, although the trunk itself still continues straight. The head joins the body at a right angle, so that the part of it which corresponds to the chin is fixed directly upon the upper part of the breast; nor can any traces of neck be discerned, until nearly the end of the second month. The inferior extremity of the vertical column, which at first resembles the rudiment of a tail becomes shorter towards the middle of the third month, and takes a curviture forwards under the rectum, in the fifth week the extremities become visible, the upper usually somewhat sooner than the lower, in the form of small blunt prominences. The upper close under the head, the lower near the caudal extremity of the vertebral column. Both are turned somewhat outwards, on account of the size of the abdomen; the upper are usually directed somewhat downwards, the lower ones somewhat upwards. [Illustration: _Diagram of the foetus and membranes about the fourth week._ _a_ Vesicula umbilicalis already passing into the ventricular and rectum intestine at _g_. _b_ Vena and arteria omphalo-meseraica. _c_ Allantois springing from the pelvis with the umbilical arteries. _d_ Embryo. _e_ Amnion. _f_ Chorion. _From Carus._]