122 C. M. CHILD. 



" head " or a single " tail." If both ends maintain their inde- 

 pendence a normal animal or double " heads" or double "tails" 

 may result according as the physiological conditions are different 

 or alike. The dorsal thickening in these cases of partial animals 

 represents that region of the body toward which the intestinal 

 contents are forced during contraction. It is caused by the 

 formation of an intestinal cavity in this region in consequence of 

 the pressure and this cavity continually enlarges in the direction 

 of least resistance, /. t\, directly dorsally (Figs. 17 and 19) or 

 dorso-posteriorly (Fig. 13) or dorso-anteriorly (Fig. 15) as the 

 case may be. As the size of the pieces from this region increases 

 normal animals are more frequently formed until beyond a certain 

 limit all pieces give rise to normal animals. 



At present I desire chiefly to call attention to the fact that the 

 two regions separated by this indifferent region resemble each 

 other in regulation since they usually produce normal animals. 

 We might expect from their position in the body to find the 

 pieces from the post-pharyngeal region predominately "pos- 

 terior" in function but as a matter of fact they show "pos- 

 terior " and "anterior" characteristics in equal degree since pre- 

 pharyngeal and postpharyngeal regions are of the same size. 

 Here again the only satisfactory explanation of the facts is the 

 existence of the early stage of individualization of a new head 

 region somewhere in this postpharyngeal region. The results 

 with small pieces indicate that this condition is not sharply cen- 

 tralized or localized in the region but extends more or less com- 

 pletely throughout its length. Moreover, the remarkable agree- 

 ment of these different lines of evidence when considered from 

 the point of view adopted in this paper affords an indication of 

 the value of this point of view as a means of interpretation and 

 unification of the mass of data which have accumulated in this 

 field. 



I believe that the supporters of theories of regulation based on 

 formative substances or upon " entelechies " will find a certain 

 amount of difficulty in interpreting some of the facts cited here, 

 but discussion of my conception of their bearing is postponed. 



As is well known, fission in Planaria inaculata occurs in a 

 region some distance posterior to the pharynx and the same is 



