THE RANGE OF DOMINANCE 141 



The different types of head in Planaria (see pp. 106- 

 14) represent, as I have pointed out, different degrees 

 of inhibition of head-formation, and, even after develop- 

 ment is completed, possess different metabolic rates, 

 as susceptibility determinations show. The metabolic 

 rate is highest in the normal head, slightly lower in the 

 teratophthalmic, and still lower in the teratomorphic and 

 anophthalmic forms. In connection with these differ- 

 ences in the heads it is of interest to note that when the 

 different forms are fed and grow, the length which they 

 attain before fission varies in general with the form and 

 metabolic rate of the head. Under ordinary conditions 

 normal animals usually become twelve or fifteen milli- 

 meters long before undergoing fission, teratophthalmic 

 forms usually slightly less, teratomorphic forms from 

 eight to ten millimeters, anophthalmic, from six to eight 

 or less, according to the degree of development of 

 the head-region, while headless forms rarely become 

 more than five or six millimeters long before dividing 

 and often divide at a length of only three or four 

 millimeters. These differences indicate very clearly 

 the difference in range of dominance associated with 

 the differences in metabolic rate in the dominant 

 region. 



There are many ways of inducing advance in develop- 

 ment of the basal zooids and the occurrence of fission in 

 Planaria, of which the simplest is the removal of the 

 head of the animal. This decreases the degree and range 

 of dominance to such an extent that fission almost 

 invariably occurs within a few days. By removal of new 

 heads as fast as they develop fission may be induced even 

 in animals much shorter than those which usually 



