"on the formation of the fœtus, where numerous problems of the following order can find a place: Why should this point be made or perfected earlier, that later? As regards the dominance of the members: Which part is the cause of the other? There are very many problems connected with the heart, such as: Why should it be the first thing (as Aristotle says in his third book on the parts of animals)[127] to acquire consistency, and be seen possessed of life, motion, and sensation, before anything has been perfected in the rest of the body? And in like manner regarding the blood: Why is it before all, and how possessed of the beginnings[128] of life and of the animal, and of the craving to move and be impelled hither and thither, to which end the heart would seem to have been made?"[129] In Harvey's celebrated treatise, despite various frank questionings by the way, such as that just quoted about the blood, he so frankly follows in the footsteps of "the master of them that know" that Aristotle need not be cited at length to prove the fact. To Aristotle are largely due Harvey's references to the heart as the central source of indispensable vital heat; his references to aliment perfected in the heart; his blending of psychological doctrines with the doctrine of the movement of the blood. Therefore, a brief account of how this became possible will be germane. When an ancient observer looked with the naked eye at the very early embryo of the fowl, he distinguished at first only a blood-red point, which pulsated, or "leapt." This Aristotle judged to be the heart, containing blood before any blood-vessel had shown itself and before blood was visible in any other part. Very soon, however, two vessels containing blood were seen, according to him, to extend from the rudimentary heart toward the periphery. From these and other considerations Aristotle inferred that both the blood and all its containing vessels owe their first origin to the heart; and that throughout life the liquid made elsewhere from the food enters the heart, there to be perfected into blood by the action of the vital innate heat, of which, as we have seen, he held the fiery central hearth to be within the heart. Naturally, therefore, he believed the blood not to be hot of itself, but to acquire its vivifying heat at the heart, the pulsation of which he held to be caused directly by the seething of the blood within. When thus perfected and charged with heat the blood, according to him, is distributed from the heart through the vena cava as well as the aorta. These great vessels and their subdivisions Aristotle distinguished anatomically; but he made no serious physiological distinction between what we call the veins and the arteries, and, himself, applied the word "artery" to the windpipe only. As to the cavities and contents of the heart, even as to the number of its cavities, he had obscure, complex, and