REGENERATION AND GROWTH 131 



wrong relation to each other. We find both processes taking place 

 in the simplest forms as in the unicellular protozoa, and present 

 throughout the entire animal kingdom without any connection, 

 excepting so far as they both depend on the general processes of 

 growth characteristic of each organ and of each animal. This leads 

 us to consider the general question of regeneration in its relation to 

 the phenomena of growth. 



REGENERATION AND GROWTH 



It has been pointed out in several cases in which external factors 

 influence the growth of a plant, or of an animal, that the same factors 

 play a similar part in the regeneration. The action of gravity on the 

 growth of plants has been long known, and that it is a factor in the 

 regeneration of a piece of a plant has also been shown. The only 

 animal in which gravity has been definitely shown to be an important 

 factor during growth is antennularia, and it has been found that 

 gravity is also a factor in the regeneration of the same form. Not 

 only is this influence shown in the growth of the new part that has 

 developed, but the same influence seems to be one of the factors that 

 determines where the new growth takes place. This latter relation is 

 known in only a few cases, for instance in plants, according to 

 Vb'chting, and in antennularia, according to Loeb, so that, until 

 further evidence is forthcoming, it is best not to extend this general- 

 ization too far ; but it seems not impossible that it may be generally 

 true. How an external factor may determine the location of new 

 growth, as well as the subsequent development of the new part, we 

 do not know at present. 



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 



