The hypertrophy, or unusual enlargement, of organs has long attracted the attention of physiologists, and the extremely interesting observations and experiments that have been made in this connection have an important although an indirect bearing on the problem of regeneration. Ribbert, as has been pointed out, holds that the processes of hypertrophy and of regeneration stand in a sort of inverse relation to each other, but it is doubtful, I think, if any such general relation exists. Two kinds of hypertrophy are now generally distinguished: functional hypertrophy, which takes place when a part becomes enlarged through use; and compensating hypertrophy, which takes place when one organ being removed another enlarges. The enlargement in the latter case may, of course, be brought about by the increased use of the parts that enlarge, but as this is not necessarily the case, the distinction between the two processes is a useful one. The causes of compensating hypertrophy are by no means simple, and several possibilities have been suggested to account for the enlargement. The best ascertained facts in connection with hypertrophy relate almost entirely to man and to a few other mammals.[51] By hypertrophy is meant an increase of the substance of which an organ is composed. Swelling due to the imbibition of water or of blood-serum is not, in a technical sense, a process of hypertrophy. Virchow distinguishes two kinds of hypertrophy: (1) Hypertrophy in a narrower sense in which the enlargement is due to an increase in the size of the cells of which an organ is composed. This enlargement of the individual cells leads of course to an increase in the size of the whole organ. (2) Hyperplasy due to an increase in the number of cells of which an organ is composed, which also causes an enlargement of the whole organ if the cells retain the normal size. The division into functional and compensating hypertrophy given above is a physiological distinction, and both of these processes might occur in Virchow’s subdivisions. Giants may be looked upon as hypertrophied individuals, since all the organs of the body are larger than the normal. The enlargement is, in this case, not due to external influences, but to some peculiarity of the organism itself. Whether the size is due to more cells being present, as seems probable, or to the cells being larger, or to both, has not, so far as I know, been determined for man. In a mollusk, _Crepidula fornicata_, in which large and small adult individuals occur, it has been shown by Conklin (’98) that the difference is due entirely to the larger number of cells in the larger individual. In this case external conditions, in so far as they retard the maximum possible growth of the individual, are responsible for the differences in size. The distinction is, in this case, rather between large normal individuals and dwarfs, than between giants and normal or average individuals.