69. The nervous system is not developed, beginning at the centre and proceeding towards the circumference of the embryo, but the reverse. Thus the lateral nerves of the head, trunk, and pelvis, are already formed, when the cerebro-spinal system is yet in a liquid state. It follows hence that those nerves cannot be considered (as it has all along been supposed) in the light of emanations from, but as distinct bodies proceeding to, that particular portion of the nervous system. (Serres.) 70. Of the apparatus for digestion, the intestinal canal is the first to appear. It consists, during the first days of its formation, of an open gutter, extending the whole length of the embryo, placed before the vertebral column, and gently curved like a canoe. It is in communication with the _vesicula umbilicalis_, or intestinal vesicle. (Wolf, Meckel, Oken, Baer.) The gutter, without losing altogether its communication with that vesicle or chamber during the first ten or twelve weeks, becomes closed at last, and extends, lengthens, and expands in a variety of ways. One of its extremities, the mouth, appears between the fourth and fifth week. It is open on the sixth. The other extremity, the anus, opens on the seventh week. About the ninth week, the outlines of a stomach are visible. (Meckel., Adelon, Velpeau.)[15] 71. The VESICULA UMBILICALIS (intestinal vesicle) corresponds to the vitelline sac of birds. The discovery is due to Bojanus. In the human embryo it measures about half an inch in diameter. It is situated immediately against the anterior surface of the embryo (Lobstein); but it gets further from it at the end of the first month, when it is found on the outside of the sheath of the cord. (Meckel.) It is composed of a granular membrane of considerable tenacity. It contains a whitish liquid, which gradually diminishes—becomes thicker, and ultimately, hardened; while the vesicle itself withers, and becomes opaque. It receives the blood vessels called omphalo-mesenteric. (Pockels.) It disappears generally about the third month. (Meckel.) It has been observed, though extremely rarely, at the full period of gestation—placed at a short distance from the insertion of the cord into the placenta, but not larger than when seen at between two and three months. (Hunter, Meckel.) It is connected, as stated before, by a short neck, and by vessels, with the smaller intestines at the termination of the ileum (Meckel)—or at the cæcum, which is, in reality, the representative of the vesicula vitellaria drawn into the abdomen as in birds. (Oken.)