Section 33. Blood, It May Be Mentioned, First Appears in the Area

vasculosa, the outer portion of the area opaca. Embryonic cells send out processes, and so become multipolar; the processes of adjacent cells coalesce. The nucleus divides, and empty spaces appear in the substance of each of the cells.

In this way, the cavities of the smaller vessels and capillaries are formed, and the products of the internal divisions of the cells become the corpuscles within the vessels. The red blood corpuscles of the rabbit, it may be added, are nucleated for a considerable portion of embryonic life. Larger vessels and the heart are burrowed, as it were, out of masses of mesoblast cells. The course of the blood in the embryo is by the veins to the right auricle, thence through the imperfection of the auricular septum already alluded to, into the left auricle. Then the left ventricle, aortic arches (for the future pulmonary artery is in communication by a part presently blocked, the ductus arterious, with the systemic aorta), arteries, capillaries, veins. The liver capillary system and the pulmonary system only become inserted upon the circulation at a comparatively late stage.