312 Morcan: HyporHesis OF FORMATIVE STUFFS 
present. Second, the amount in the stem does not decrease, 
while that at the end is increasing ; and third, the red pigment of 
the circulating fluid disappears, not by going into the forming 
wall of the new hydranth, but by being collected into a ball in its 
stomach. Stevens has shown that this ball is later vomited from 
the mouth. Thus the pigment hypothesis of tubularia has been 
disposed of. Let us now consider the experiments that bear di- 
rectly on my main theme. I have just pointed out that two heads 
develop on pieces of the stem of tubularia, one twenty-four hours 
after the other, This delay in the appearance of the proximal head 
would be interpreted on the stuff-hypothesis as due to the distal 
head, that first develops, using all the available hydranth-forming 
material. It can be shown that this cannot be the explanation. 
First, if a thread is tied around the middle of the piece the devel- 
opment of the distal hydranth is not delayed, but the development 
of the proximal hydranth is greatly hastened. Second, if a piece 
is simply bent, the proximal head will develop almost as soon as 
the distal one. Third, short pieces (if not so small that the usual 
kind of development is interfered with) produce distal heads as soon 
as do longer pieces ; thus showing that the amount of material in 
a long piece is more than ample to allow a hydranth to develop at 
each end at the same time. Fourth, if the two ends of a long 
piece are allowed to close, and if then the piece is cut in two, the 
development of the distal hydranth is hastened. We may con- 
clude from these experiments that the delay in the appearance of 
the proximal hydranth is not due to the direction of the flow or 
the formative substances, because when a long piece is simply cut 
in two the development of the proximal hydranth may be hastened. 
The delay is due, only in a very small degree, to the polarity, 
because if the piece is simply tied in the middle the development of 
the proximal hydranth is hastened. Neither can the delay be due 
to the amount of food-stuff available, because pieces much shorter 
than half of these long pieces produce distal hydranths as soon 
as do the longer pieces. The stuff-hypothesis fails to explain the 
facts from every point of view. 
The results certainly suggest that some physical factor enters 
into the problem. It seems to me that whatever this physical 
factor may be, it is the same that we find acting wherever growth 
and the formation of new structures are taking place. 
