268 Experimental Zoology 



Thus the stolons of "hydroids and of bryozoans of some of the 

 compound Ascidians" cHng 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 a new 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, i.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 oj Galls 



The formation of galls is a 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-hke 

 sweUings to appear. Perhaps the cyst on fish caused by the para- 

 sitic larvse of the fresh- water mussel, Anodonta, may also be con- 

 sidered a gall, as well as the cysts found about parasitic trichinae, 

 chigoes, etc. 



The galls that have been most studied are those on plants, 



* See Loeb (1892). 



