Having obtained these data, let us see how the ampulla is constructed in which the transformation will take place. On the 8th of July 1906, my son Paul, my zealous collaborator now that my once sturdy legs are failing me, brings me, on returning from his morning walk, a magnificent branching head of mullein peopled by the Cionus. It contains an abundance of larva. Two of them in particular delight me: while the others stand browsing, these two wander about restlessly, indifferent to their food. Beyond any doubt, they are looking for a spot favourable to the process of the nymphosis. I place each of them singly in a small glass tube which will allow me to observe them easily. In case they might find the food-plant useful, I supply them with a sprig of mullein. And now, lens in hand, from morning to evening and then by night, as far as drowsiness and the doubtful light of a candle will permit, let us be on the alert; for very interesting things are about to happen. Let me describe them hour by hour. 8 A.M.—The larva is not making use of the twig with which I provided it. It is crawling along the glass, darting its pointed head now this way, now that. With a gentle creeping movement that causes an undulation of the back and belly, it is trying to settle itself comfortably. After two hours of this effort, which is certain to be accompanied by an emission of viscous fluid, it finds a position to its taste. 10 A.M.—Being now fixed to the glass, the larva has shrunk into the semblance of a little barrel, or a grain of wheat with rounded ends. At one end is a shining black speck. This is the head, jammed into a fold of the first segment. The grub’s colour is unchanged: it is still a dirty yellow. 1 P.M.—A copious emission of fine black granules, followed by semifluid dejecta. To avoid soiling its future residence and to prepare the intestine for the delicate chemistry about to follow, the grub purges itself beforehand of its impurities. It is now a uniform pale yellow, without the cloudy markings that disfigured it at first. It is lying at full length on its ventral surface. 3 P.M.—Under the skin, especially on the back, the lens reveals subtle pulsations, slight tremors, like those of a liquid surface on the point of boiling. The dorsal vessel itself is dilating and contracting, throughout its length, more actively than usual. This means a fit of fever. Some internal change must be preparing, which will affect the whole organism. Can it be the preparation for a moult? 5 P.M.—No, for the grub is no longer motionless. It leaves its heap of dirt and begins to move along impetuously, more restlessly than ever. What is happening that is in any way unusual? I think I can obtain some idea of it with the aid of logic.