In general it may be stated that the limits of temperature under which normal growth may take place represent also the limits of temperature for regeneration. Lillie and Knowlton (’97) have determined the limits of temperature within which regeneration takes place in _Planaria torva_. The worm was cut in two transversely through the pharynx, and the time required at different temperatures to produce a new head on the posterior piece was recorded. The lowest temperature at which regeneration was found to take place was 3°C. Of six individuals kept at this temperature only one regenerated at all, and in this one the eyes and brain were still incomplete after six months. The optimum temperature, or at least that at which regeneration takes place most rapidly, was found to be 29.7°C.; a new head developed in 46 days at this temperature. At 31.5°C. regeneration was slower, requiring 8.5 days to make a new head. At 32°C. incomplete regeneration sometimes took place, but death occurred in about six days. At 33°C. regeneration was very slight, and the animals died within three days. At 34°C., and above this point, no regeneration took place, and death soon occurred. In _Hydra viridis_, Peebles (’98) has found that regeneration is quicker at 26°-27°C. than at 28°-30°C. At the former temperature regeneration takes place in 48 hours. If kept at 12°C. pieces may regenerate in 96 hours, but not all the pieces had regenerated in this case until 168 hours. _INFLUENCE OF FOOD ON REGENERATION_ While the growth of an animal or of a plant is, in most cases, and, of course, within certain limits, directly connected with the amount of food that is obtainable, nevertheless extensive regeneration may take place in an animal, or part of an animal, entirely deprived of food. In this case the material for the new part is derived from the excess of material in the old part, and not only surplus food material, but even the protoplasm itself appears to be drawn upon to furnish material to the new part. The relation between regeneration and the amount of food present in the old part is well shown by experiments with planarians. If a planarian is kept for several months without food, it will decrease very much in size. In fact, the volume of a starved worm of _Planaria lugubris_ compared with that of a fully fed individual may be only one-thirteenth of the latter (Fig. 13, _A_, _B_). If a starved worm is cut in two pieces, each piece will regenerate, although less quickly than in a well-fed worm. The new part will continue to increase in size at the expense of the old piece that is already in a starved condition. On the other hand, an excess of food does not necessarily produce a hastening of the regeneration, for, as Bardeen (’01) has shown, worms that have been for several days without food may regenerate more quickly than worms that have been fed just before they were cut into pieces.