Physiology of Regeneration 637 
sure. Perhaps there is some other factor responsible for the 
acceleration. In fact, Miss Peebles (’08), in a recent paper on 
grafting experiments with Tubularia, pointed out that her experi- 
ments on the influence of concentration of the sea-water yielded 
results which indicate “that the great increase in size of the 
hydranths and the rapidity of their formation in dilute sea-water 
is due to something more than the difference in osmotic pressure.” 
She believes that there are some organic substances in the sea- 
water which exert a retarding effect upon regenerating stems of 
Tubularia, and that this effect may be abolished by diluting the 
sea-water, so that “the growth which we consider so unusual 1s 
really no more than the normal rate under optimum conditions.” 
This suggestion is in a sense in accordance with the results of 
my experiments, where, as we have seen, KCl and MgCl, pro- 
duced opposite effects upon regenerating worms in virtue of their 
different chemical properties, since the osmotic pressure was the 
same in both cases. It is likely, therefore, that MgCl, produces a 
favorable effect on the rate of regeneration when present in the 
optimum quantity, as do other substances employed 1 in the pre- 
ceding experiments, because of its special properties and not 
through a change in osmotic pressure. 
VIII COMPENSATION 
It was remarked in the beginning of this paper that the worms 
have a chitinoid covering, usually of a seal-brown color. When 
a new tail begins to grow out this covering is at first lacking and 
the tail is therefore more or less translucent. The new tail soon 
acquires this chitinoid covering, but while the covering is being 
formed here, it gradually disappears from the old piece. In Fig. 
7 an attempt is made to represent this process diagrammatically. 
In this figure (1) is supposed to represent the condition when 
the old piece is thickly covered over, and the new tissue is still 
“naked.” From this time on, the covering on the old part changes 
from dark brown to light shades of yellow. The change may 
first take place either on the front portion of the animal (3) or 
on the portion nearest to the regenerating tissue. (2) or occasionally 
