About the second week, or perhaps the third, there is a mass of globules loosely connected together forming the _primitive streak of Von Baer_, and around this the _area vasculosa_ is developed. The globules of the primitive streak, seem next to be developed into two laminæ dorsales, which is the axis of the future embryo, and the origin of the spinal column. That portion of the fluid that separates the _chorda dorsalis_ from the lamina dorsalis is the future spinal cord, and brain. Two other lamina—_laminæ ventrales of Von Baer_—are in the mean time proceeding from the axis of the embryo, one on each side; they grow laterally and converge below the axis. After the rudiments of organic life have been commenced in the central portion of the serous layer, a fold of its peripheral portion arches over the dorsal surface of the embryo so as to represent a sac, and is composed of two membranes; the one next to the fœtus is the _amnion_, the other is gradually separated from the amnion and joins the serous lamina of the blastoderma; this is the _false amnion_ of Pander. The heart is formed at this early period, and although there is no septum between the ventricles, a vein may be seen entering into it, and an artery passes out which divides into four branches to be distributed and ramified in different portions of the fœtus. The abdomen is yet an open cleft, in which (but projecting beyond it) is the heart, which is of very large dimensions; behind the heart is the liver, and under the liver the intestine which is attached by means of a distinct mesentery. At this period (three weeks) the lungs are constituted of five or six lobules, and two large glandular structures may also be discerned along the vertebral column, which are called Wolfian bodies; these anticipate the function of the kidneys. The _alantois_ is seen arising from the lower part of the intestinal canal on a little vesicle and extending so as to encircle the embryo. During the second month the extremities are growing, and become more projecting; the body is curved and the head bent downwards; there are indications of the nostrils and a gaping oral aperture; the forehead is more swelled because of the development of the hemispheres of the brain; the spinal cord is cylindrical of nearly uniform thickness and terminating in a blunt extremity—posteriorly it is open; the _medulla oblongata_ makes a bend forwards at the top of the neck, and then ascends perpendicularly into the capacious cranium.