296 Experimental Zoology 
plied with a mouth opening. The reversal of polarity described 
for reversed grafts of hydra has been discovered by Mrs. Mor- 
gan to take place in planarians also when the piece is very short. 
If, for instance, anterior cut surfaces of two pieces are united, 
and one of the pieces is subsequently cut off near the line of 
union, that piece if very short produces a head at its posterior 
exposed end. Here also it appears that the results may be due 
to the influence of the old part. In this case there is no question 
of absorption of the smaller piece, but there is on the otherhand 
ample opportunity for inwandering cells to pass from the large 
part through the small piece into the new material. The result 
is further complicated by the fact that very short pieces of other 
species of planarians, that are not grafted, may produce a head 
at the anterior end and another at the posterior end, and while 
at present this result has not been obtained with Plagocata, the 
possibility still exists that the results may be due to the cut-off 
pieces that have been tested for double heads being somewhat 
longer than the small grafted piece in the reversed position. If 
the grafted piece is longer, it produces a tail at its posterior end 
and not a head. 
Miss Peebles has shown in Tubularia that small pieces grafted 
on to the ends of large pieces may sometimes take part in produc- 
ing the single head that develops. This may occur also when 
the small piece is reversed in direction; but it cannot be shown 
in this case that the reversed head is due to its union with the 
larger piece, because Tubularia produces a new head so readily 
from either the aboral or oral end that even if some influence 
of the larger piece exists it would be difficult to prove. 
The peculiar power of pieces of hydra, planarians, etc., to 
mold themselves into a new form of typical proportions is clearly 
similar to the molding that takes place in embryonic develop- 
ment. The older writers used the term “formative force” 
to account for this power to undergo changes in form, but 
modern investigators avoid the use of this term, because no such 
form of energy is known in the physical world; and because we 
should have to postulate as many kinds of formative forces to 
