62. The growth and progress of the Embryo or Fœtus follow a very irregular march. Up to the second month the increase is somewhat slow—it is accelerated during the third—it slackens again at the fourth and fifth months—between which and the last month the increase is more rapid, until it has acquired its proper maturity. (Autenrieth, Soemmering.)[11] 63. The Embryo may be perceived, with the naked eye, at the fourteenth day after conception. It measures then 1–12th of an inch in length. (Dr. Pockels.) On the third week, it is 1–10th of an inch long. It is as large as a house-fly at four weeks, and as a horse-fly at six weeks. At two months it weighs twenty grains, and is one inch long. It weighs an ounce and a half at three months, and measures three inches; between which time and the sixth month it increases in dimensions from three to nine inches, and in weight from one ounce and a half to one pound. The relative weights of augmentation of length for the seventh, eighth, and ninth months stand thus:—from two to four pounds, and twelve inches; from four to five pounds, and seventeen inches: from five to eight pounds and twenty-two inches. (Averages of minute and accurate observations made by Autenrieth, Soemmering, Bichat, Pockels, Carus, &c.—confirmed by my own observations made on several early ova, and many fœtuses examined in the course of seventeen years obstetrical practice.) 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.)