In studying a machine one first seeks the most important part, without which it could not move nor work. In the mechanism of our body, the first part to develop and move is the heart, and it likewise is the last to stand still. The development of this organ may be better studied in the hen’s egg than in any other animal, as it can be seen on the second day of incubation. At its first appearance it is in man, as in animals, a fine, curved tube in the shape of an =S=. If we break an egg taken from under a brood-hen towards the end of the second or the beginning of the third day, the first rudiment of the heart may already be seen pulsating. Towards the end of the fourth week after conception, the human heart has already nearly the form which it preserves during the whole of life. It is wonderful with what resistance the heart struggles on its first appearance against every cause which threatens its life. Professor Pflüger relates that a human embryo of about three weeks’ gestation was left a whole night between two watch-glasses in a cold room. In the morning it was found that the little heart still contracted at intervals of twenty to thirty seconds, and these movements were noticeable for almost another hour, becoming gradually slower and weaker until the complete death of the embryo. In animals incompletely developed there is no emotion capable of modifying the rhythm of the heart. In a series of experiments which I made on the heart in a hen’s egg during the first days of its development, I found that the application of the strongest inductive currents, such as were unbearable on the hands, produced not the least effect. It was a strange sight, this surprising tenacity and unexpected resistance in a little heart which was scarcely visible, and which pulsated tranquilly under electrical discharges which would have killed instantaneously the heart of a horse or an ox. This shows us how well the organs are adapted to their functions. It is the task of the heart in the chicken to work blindly and incessantly in order to bring into circulation the little particles which gradually build up the body of the animal, using for this purpose the materials accumulated in the egg after it has received the spark of life through fecundation. In the embryo there is no need to receive the impressions of the outside world, and the organs for this purpose are still lacking, the nerves have not yet appeared, the heart is free in the midst of the chaos of matter in the course of organisation. III