MorGan: Hyporuesis oF ForMATIVE STUFFS 211 
its development? But if this is the case the difference in the 
behavior of the distal and proximal buds still remains to be ac- 
counted for, and this is the main point now under discussion. If 
we should be tempted to supplement the first assumption by 
making use once more of the idea suggested above in another 
connection, namely, that the youngest buds will be the first to 
develop, we should still, as we have already seen, get into a diffi- 
culty in regard to the roots. 
Iam of the opinion that the phenomenon of regeneration can _ 
not be explained by a purely chemical hypothesis. I suspect, at 
least, that certain physical conditions may be equally important. 
My meaning can be best illustrated by certain experiments that I 
have recently made with the tubularian hydroid, 7wbularia Mesem- 
bryanthemum. This case is especially interesting in the present 
connection, because it is the only instance in modern times in 
which the hypothesis of formative stuffs has been applied to ani- 
mals to explain regeneration. Moreover, hydroids react in many 
ways to external stimuli much like plants. 
If long pieces of the stem of tubularia are cut off, and then the 
head or hydranth be removed from the distal end, it will be found 
that a new head develops in the course of twenty-four hours. 
This head appears at the distal end of the piece. After another 
twenty-four hours another head develops on the other, the prox- 
imal, end of the piece. 
The stem as well as the hydranth-head of tubularia contains a 
red pigment. Before a new head develops at a cut end the same 
red pigment begins to appear in this region. Loeb suggested that 
this pigment is a formative stuff produced by the stem, and that its 
accumulation at the anterior end gives the stimulus for the forma- 
tion of a new hydranth. Later Driesch also studied the regener- 
ation of these pieces of tubularia, and observed that red pigment 
appears in great quantity in the fluid that circulates in the center 
of the piece. He thought that its presence might act quantita- 
tively, and determine the kind of incomplete structures that short 
pieces of the stem often produce. 
I have been able to prove that both of these suggestions are 
wrong, because in the first place the amount of pigment that de- 
velops in small pieces is much greater than that which was at first 
