282 Experimental Zoowgy 
ing here, no doubt, with an extremely elusive and difficult prob- 
lem, but one that is of fundamental importance in all 
questions in which the problem of organic form appears. It 
may be that there are factors at work here of which as yet we 
have little real conception, and any such attempt as this that I 
have made to give the facts an explanation on more or less famil- 
iar assumptions may be premature. I hope, nevertheless, that 
I may have succeeded in calling attention to certain important 
phenomena of growth, and even if my attempt to bring the re- 
sults under one point of view should prove unsatisfactory, the 
attempt may at least serve as a sHEeeuOn for further work 
along these lines. 
To sum up: I have attempted to account for certain phe- 
nomena of regeneration by a process of growth in which the 
following factors appear to enter: (1) the differentiated material 
as a factor in limiting the character of new parts; (2) the relation 
of the cells to each other as a factor in their differentiation, and 
assume that this relation is due to the mutual pressures or 
tensions of the cells on each other; (3) the differentiated cells 
also determine the existing tension in that part, and this may 
in turn react on the new cells with which they are in contact. 
Remove a part and the pressure relations are upset, but this 
leads ultimately to the reéstablishment again of the same rela- 
tions of pressure. 
LITERATURE, CHAPTER XVII 
Kine, H. D. Further Studies on the Regeneration of Asterias vulgaris. 
Arch. f. Entw.-mech, IX. 1900. 
Morean, T. H. Regeneration. Igor. 
The Physiology of Regeneration. Jour. Exp. Zool. III. 1906. 
pie ea to L. Prodromo di un’ opera sopra le Riproduzioni animali. 
1826. ~ 
WEIsMANN, A. Essays upon Heredity. Trans. 1891-1892. 
ZELENY, C. A Study of the Regeneration of the Arms of the Brittle-Star. 
Biol. Bull. VI. 1903. 
Compensatory Regulation. Jour. Exp. Zool. II. 1905. 
