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]