I have always said, that the organic molecules were fixed, and that their uniting was caused only by their loss of motion. This appears to me certain: for, if we separately examine the seminal liquor of the male and female, we shall see an infinity of small bodies in great motion, but being mixed, their motion is instantly suspended, and heat is necessary to renew their activity; for the chicken which exists in the centre of the cicatrice is without any motion before incubation; and even twenty-four hours after, when it begins to become perceptible with a microscope, there is not the least appearance of motion then, nor even the day following. During the first day it is only a small white mucilaginous mass, which is of a consistence on the second, and insensibly increases, but whose motion is very slow, and does not at all resemble that of the organic particles which move rapidly in the seminal liquor. Besides, I have reason to say, that this motion of the organic molecules is absolutely destroyed; for if we keep an egg without exposing it to a degree of heat necessary to expand the chicken, the embryo, although formed entirely, will remain without any motion; and the organic molecules of which it is composed, will remain fixed without being able to give motion and life to the embryo which has been formed by their union. Thus, after the motion of the organic molecules has been destroyed, after the union of these molecules, necessary to form an animal body, there is still an external agent required to animate and give it life and motion; and this agent is heat, which, by rarefying the liquors, obliges them to circulate and put also every organ in action, which afterwards do no more than develope and grow, provided that this external heat continues to assist them in their functions. Before the action of this external heat, not the least appearance of blood is to be seen; and it is not till twenty-four hours after, that I have perceived any change in the colour of the vessels. The blood first appears in the placenta, which communicates with the body of the chicken: but this blood seems to lose its colour as it approaches the body of the animal; for the chicken is entirely white, and we with difficulty discover in the first, second, and third days after incubation, a few small sanguinary points which are close to the body of the animal, but which seem not to make part of it, although it is these sanguinary points which afterwards form the heart. Thus, the formation of the blood is a change occasioned in the liquors by the motion heat communicates to them, and this blood is formed even out of the body of the animal, the whole substance of which is then only a kind of mucilage, or thick jelly.