CONTROL OF HEAD FORMATION IN PLANARIA 37 



activity of the Y region is low, or its stimulation by section is 

 slight, or is inhibited by some agent, the stimulated X cells are 

 freed to a greater or less extent from any integrative control or 

 the controlling factors emanating from Y are not adequate to 

 coiitrol them completely. 



It is a well-known fact that an isolated group of cells from the 

 body of a plant or many of the lower animals undergoes dedif- 

 ferentiation and development into a new individual if it is not 

 made up of highly specialized tissues and is able to maintain 

 life. The well-known facts of agamic reproduction constitute 

 evidence beyond question that isolation is followed by the changes 

 attendant upon the development of a new individual from an 

 isolated part. The very fact that a piece taken from the body 

 of a planarian will reconstitute a new individual in many cases 

 like those in nature is itself a demonstration of the accuracy of 

 the statement. These processes of development are initiated 

 at the point where the metabolic activity is highest. The point 

 of highest rate in a piece of Planaria is the X region. (For a 

 full discussion and references on the subjects of isolation and 

 the establishing of the new individual, see Child, Individuality 

 in Organisms, Chicago, 1915.) 



There are, therefore, two factors that are plainly antagonistic 

 exerting influences on the X cells, one, the factor originating 

 in Y which tends to prevent the dedifferentiation of X, the other 

 tending to bring about dedifferentiation, cell division, growth, 

 and the initiation of development of a new individual. If either 

 of these two factors is inhibited or is inhibited more than the 

 other, the one inhibited less becomes the factor which more or 

 less dominates the fate of the X cells. Consequently, if a single 

 piece is isolated from an animal, whether or not a head forms 

 will depend on the relative activities of these two factors. And 

 in a mass experiment when the pieces are subjected to experi- 

 mental conditions, the head frequency is increased or decreased 

 in relation to the differential effect of the agent or condition 

 on these two factors. The two antagonistic factors are undoubt- 

 edly functions of the rates of metabolism of the X and Y regions, 

 respectively, and may be conveniently described as Rate X and 

 Rate Y, as pointed out above (p. 7). 



