268 Experimental Zoology 
Thus the stolons of “hydroids and of bryozoans of some of the 
compound Ascidians” cling to a substratum with which they 
have come into contact and refuse to leave it, following all of 
its irregularities. Even the under surface of the film on the top 
of water will call forth this response. 
In the development and regeneration of new parts from a 
piece of the stem of Tubularia a response to contact can easily 
be demonstrated. If anew hydranth of Tubularia on emerging 
from the stem comes into contact with a solid body, it turns away 
from it and grows at right angles to the surface of contact. If, 
again, the basal end of a piece of the stem of Tubularia comes in 
contact with a solid, it develops a stolon (and not a hydranth). 
On the other hand, the oral: end if brought into contact with a 
solid develops a hydranth, 7.e. it does not respond to contact. 
Other hydroids, however, will develop a stolon at the oral end 
if this touches a hard surface. I have suggested that the con- 
tact reaction between the cells in different parts of the body may 
be one of the important factors in determining not only the 
molding of the form of each organ during the development, but 
that the pressure relations of the parts may be an important 
factor in their growth. 
The Formation of Galls 
The formation of galls isa remarkable phenomenon of growth, 
for galls are well-defined structures, differing from anything else 
that the plant normally produces. The best-known galls are 
those found on the higher plants and are caused mainly by in- 
sects. A few gall-like growths also occur on animals, as in the 
case where certain crustacea infesting corals cause gall-like 
swellings toappear. Perhaps the cyst on fish caused by the para- 
sitic larvee of the fresh-water mussel, Anodonta, may also be con- 
sidered a gall, as well as the cysts found about parasitic trichine, 
chigoes, etc. 
The galls that have been most studied are those on plants, 
1See Loeb (1892). 
