"Therefore, relying with certainty upon what I have observed in the egg and in the dissection of living animals, I maintain against Aristotle that the blood is the primary generative part; and that the heart is its organ, destined to send it on a circuit. Surely the function of the heart is the propulsion of the blood, as is admirably clear in all animals that have blood; and in the generation of the chick the same duty falls to the pulsating vesicle, which in the very early embryos of animals[178] no less than in the egg I have often exhibited to view as something more minute than a spark, beating and when in action contracting itself and at the same time pressing out the blood contained in it, and in its relaxation receiving the same afresh."[179] Whether in studying the foregoing passages we read Harvey's earlier jottings in his private note-book or the deliberate statements published in his old age, it is evident that to his mind the question of the primacy of the blood versus the primacy of the heart depends for answer upon the further question whether in the development of the embryo the blood be made before the heart, or the heart before the blood. In no other part than one of these two can the primacy inhere, for him; and whichever of these two has the priority must be, to Harvey's mind, the origin of the other and of the remaining parts and must continue to be the "principal part" of the body throughout life. The matter of the primacy thus resolves itself into one of well-devised and accurate observation; and the discoverer is once more upon the ground where his undying laurels grew. He, therefore, deals no longer "without demonstration," as in the second Exercise to Riolanus, but makes report of actual observations and so gives ocular evidence in support of his views, remembering, it may be, that he had said to Riolanus: "Soon, perhaps, I shall make public things even more wonderful and destined to cast even greater light on natural philosophy."[180] Harvey's contemporary Milton said to Parliament: "Truth is compar'd in Scripture to a streaming fountain; if her waters flow not in a perpetuall progression, they sick'n into a muddy pool of conformity and tradition."[181] These words seem timely as we note the great discoverer, magnifying glass in hand, searching in incubated eggs for an answer to the question, now wholly obsolete, whether the primacy of the heart should not give way to the primacy of the blood.