The experiments described in Chapters II, III, IV, have shown that there are many resemblances between the phenomena of growth and of regeneration. It has been pointed out that when it could be shown that certain external agents have a determining influence upon growth, these same agents have a similar effect upon regeneration. This also holds apparently for internal factors, although it is much more difficult to demonstrate that this is true. The presence of an abundance of food material in the tissues hastens regeneration in the same way that growth is more rapid in a well-fed organism. Food may, however, be looked upon rather as an external factor than as an internal one. An excellent example of an internal factor is found in the interrelations of the parts to each other. This is shown in the development of a piece of a plant in which the apical buds develop faster than the proximal ones, and it appears that, in some way, the development of the latter are held in check by the development of the apical ones. Another case is found in the development of the bilobed tail of certain fish in which particular regions are held in check, while others grow at the maximum rate. It is a curious fact that while we can cite several kinds of external influences that affect the development and the regeneration of organisms, the only internal factors that have been discovered are the so-called polarity and this interrelation of the parts. Perhaps there should also be added the specific nature of certain parts, limiting the possibilities of new growth in these parts, and the presence of the nucleus as necessary for the growth and regeneration of the organism.