64. Towards the end of the first month it is easy to distinguish the head of the fœtus, which has the appearance of a vesicle composed of thin membranes, and is as large as the rest of the body. The latter is straight—and there is in one part of the head a black dot, marking the spot for the eyes. (Plate I. fig. 5.)[[12]] But it is during the second month that the various parts of the face, as well as the superior and inferior extremities, become visibly delineated in the fœtus. The eyes are indicated by two black spots of considerable size, compared to the volume of the fœtus. The eyelids are not spread over them until the tenth week, at which time the external concha of the ear, as well as the profile form of the nose, become properly defined. At this same period we find the mouth, which was at first a simple opening of no mean size, becoming more distinctly delineated, owing to the formation of the lips. These soon approach each other, leaving a simple transversal slit or depression between them. (Plate II. fig. 11 and 12.)
65. The extremities appear in the form of globular tubercles at first; the superior preceding the inferior by a short period of time. Until between the third and fourth month, the lower are smaller than the superior extremities. Even at the latter month they are shorter than the projection of the os coccygis. The parts of reproduction may then be distinctly seen. In the male as well as in the female, these parts are exceedingly prominent. (Plate V. fig. 18 and 19.) But the difference of the sexes may be deduced from other circumstances besides the sexual organs. Such as the particular structure of the thorax and abdomen, the form of the head, the extremities, and the dorsal spine. (Autenrieth, Soemmering, Meckel, Velpeau.)
66. The external organs of the fœtus are all well defined at the end of the fourth month. The abdomen is fully covered in; the intestines are no longer visible. By the sixth month the fœtus is perfect in its shape and formation. In the seventh, eighth, and ninth months, its successive development is limited to mere size, volume, or amplitude, and a proportionate increase in weight. It is well to keep in mind, that although the growth of the various parts of the fœtus bears a proportion to the general development of its body, that part of the body which is above the navel, measures in length more than the part below it, until the full and complete period of gestation, when the navel marks the precise middle of the length of the fœtus. This circumstance assists us materially in forming our opinion respecting the age of any fœtus, either as a matter of curiosity, or as a guide in questions of forensic medicine. (Foderé, Chaussier.)
67. It was supposed, until within the last fifteen years, that the nervous system was the first to be formed in the human embryo; but recent discoveries have proved that the vascular system has precedence of the former. The nerves invariably appear after the arteries which they are intended to accompany. The spinal marrow appears before the brain, and the latter before the cerebellum, and accordingly it is found that the arteries of the spinal marrow shew themselves before those of the brain, and the arteries of the brain become manifest before those of the cerebellum. In this respect the organogenesy of the human fœtus is analogous to that of birds. (Serres, Desmoulins, Adelon.)[[13]]
68. The blood is formed independently of the heart, and appears at two distinct points from it, and acquires a motion independently of it. (Prevost, Dumas, Baer.) The veins are formed first—next, the heart—lastly, the arteries, &c. (French Physiologists.) The arteries are, by an Italian physiologist, said to be the first to appear. (Rolando.)[[14]]
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.)
72. At the lower end of the anal intestines, there projects another vesicle or sac, to which the name of Allantoid has been given. It exists in birds—in all the mammalia—and in the human embryo. In the latter it appears about the fourth week, and by the sixth week it has almost disappeared. Its existence is therefore but short, while the size it attains is always trifling. Not so with the mammalia, in which it acquires great amplitude. The communication of this sac with the bladder, by a canal called the urachus, and which canal is found impervious after the first three or four months of gestation, is admitted by all observers. (Meckel, Baer, Dutrochet.)
73. The umbilical cord or navel string, in man, has been found to spring from a vesicle to which the discoverer has given the name of Erythroid. (Dr. Pockels.) This vesicle had been before observed in the mammalia. (Oken.) The cord appears at the end of the third week. (Pockels.) It then consists of a vein and two arteries, the urachus, a species of gelatine of a ropy nature, called the gelatine of Wharton, a portion of the intestinal canal, (larger in proportion as the embryo is younger,) the vesicula umbilicalis, in part, and the omphalo-mesenteric vessels, the whole enveloped by a sheath proceeding from the involucra of the ovum already described. The last three constituent elements of the cord, however, become obliterated or disappear after the third or fourth month of gestation—the others remain permanent throughout that period.