In regard to the internal factors that influence the growth and the regeneration of new parts, we are almost completely in the dark. In cases of hypertrophy of the kidney, etc., the evidence seems to show that a specific substance, urea, that is normally taken from the blood by this organ may, if present in more than average amounts, excite the cells to greater activity and to growth, but whether the urea itself does this directly, or only indirectly through the greater functional activity of the cells, has not, as we have seen, been ascertained. That growth is influenced by internal factors can be shown, at least in certain cases, even although we cannot refer to the definite chemical or physical factors in the process. Some experiments that I have made on the tails of fish show very clearly the action of an internal factor. If the tail of fundulus is cut off obliquely, as indicated by the line 2-2 in Fig. 40, _A_, new material appears in a few days along the outer cut-edge. It appears to be at first equal in amount along the entire edge. As the material increases in width, it grows faster over [Illustration: FIG. 40.--_A._ Tail of _Fundulus heteroclitus_. Lines indicate levels at which _B_ and _C_ were cut off. _B._ Regenerating from cross-cut. _C._ Regenerating from oblique cut. _D, E._ Regenerating from two oblique surfaces. _G._ Tail of stenopus. _H, I._ Tail of last cut off squarely and obliquely.] that part of the edge that is nearer the base of the tail (Fig. 40, _C_). This growth continues to go on faster on the lower side, until the rounded form of the tail is produced. If we make the oblique cut so that the part nearer the base of the tail is on the upper side, the result is the same in principle; the upper part of the new material grows faster than any other part. If we make two oblique cuts on the same tail, as shown in Fig. 40, _D_, or as in _E_, the new part grows faster in each case on that part of the cut-edge that lies nearer the base of the tail. These results may be supposed to be due to the better nourishment of the new tissues nearer the base of the tail; but it is not difficult to show that the difference in the rate of growth over different parts of the cut-edge is not due to this factor. If, for example, we cut off the tail of one fish squarely near the outer end, as shown in Fig. 40, _F_, 1-1, and the tail of a second near the base of the tail, as shown in Fig. 40, _F_, 2-2, and of a third by an oblique cut that corresponds to a cut extending from the upper side of the cut-edge of the tail of the first fish to the lower cut-edge of the tail of the second fish, as shown in Fig. 40, _F_, we find that the rate of growth over the first and second tails is about the same as that of the lower side of the third tail. In other words, the maximum rate of growth that is possible for the entire oblique edge is carried out only near the lower edge, and the growth of the rest of the new material is held in check. By means of