The experiments made by Ponfick (’90) on the regeneration of the liver in dogs and in rabbits gave the most striking results. Ponfick found after removal of a fourth, or of a half, or even, in a few successful operations, of three-fourths of the liver, that, in the course of four or five weeks, the volume of the remaining part increased, and in the most extreme case, to three times that of the piece that had been left in the body. The first changes were found to have begun as early as thirty hours after the operation, when the liver cells had begun to divide. The maximum number of dividing cells was found about the seventh day, and then decreased from the twentieth to the twenty-fifth day, but cells were found dividing even on the thirtieth day. These dividing cells appeared everywhere throughout the liver, and were no more abundant at the cut-edges than elsewhere. There takes place, in consequence, an increase in the volume of the liver, rather than a replacement of the part that is removed. The increase takes place in the cells of the old part, the lobules swelling up to two, three, or even four times their former size. No new liver lobules seem be formed. The old tubules of the liver also become larger, owing to an increase in the number of their cells. Since the change takes place in the old part, and is due to an increase in size of the lobules, tubes, etc., the process is spoken of as one of hypertrophy rather than of regeneration. Kretz found a case in which the entire parenchyma of the liver seemed to have been destroyed, presumably by a poison from some micro-organism, and later a regeneration of the tissue had taken place. If this conclusion is correct, it shows that sometimes an internal organ may meet with an injury that does not directly destroy the rest of the body, and the animal may survive. The regeneration of the salivary gland of the rabbit described by Ribbert is another example of an internal organ that can seldom be injured, and yet can be replaced after artificial removal. Weismann (’93) has recorded an experiment in which half of a lung of triton was cut off. After fourteen months the lung had not been restored in four individuals, and in one “it was doubtful whether a growth of the lung had not taken place, but even in this case it had not recovered its long, pointed form.”