PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 1 39 



a day or two, and- then gradually rises as reconstitution progresses. 

 Curve // shows the susceptibility of fourths left in water fifteen 

 to eighteen hours after section and then placed in KNC ; curve gg, 

 that of sixths under the same conditions ; curve hh, that of eighths 

 after forty hours in water. And, finally, curve it shows the sus- 

 ceptibility of young growing animals 5 mm. long raised from 

 pieces — i.e., approximately the susceptibility attained by fourths 

 after reconstitution is completed — that of sixths and eighths being 

 somewhat higher. 



Some part of this increase of susceptibility after section is un- 

 doubtedly due to the presence of one (bb) or two {cc, dd, ee) cut 

 surfaces. Other experiments for which curves have been plotted,, 

 but which are not shown here, demonstrate that pieces with 

 oblique and therefore larger cut surfaces are more susceptible than 

 pieces with transverse surfaces. 



The experiments on susceptibility show one other point of im- 

 portance which does not directly appear in Fig. 24 and which 

 would require a number of graphs for full presentation. It was 

 stated above that pieces of the same length from different levels 

 show approximately the same susceptibility. In the section on 

 susceptibility gradients it was shown that susceptibility decreases 

 from the anterior to the posterior end of the first zooid and in- 

 creases again with approach to the level of the posterior zooid. 

 Even if the susceptibility of pieces from all levels were approxi- 

 mately the same immediately after section, it would be evident that 

 the increase in susceptibility must have been much greater in pieces 

 from posterior than in those from anterior levels of the first zooid 

 and from the posterior zooid. It follows from this fact that sus- 

 ceptibility is not simply a matter of the presence of cut surfaces, 

 but depends in part upon level of the body from which pieces are 

 taken. 



Comparison of these changes in susceptibility following section 

 with those in P. dorotocephala? brings to light some interesting 

 physiological differences between the two species. In the first 

 place, removal of the head does not appreciably increase suscepti- 

 bility in P. dorotocephala. In fourths it is increased only slightly, 



* See Child, '14a. His results have been repeatedly confirmed by myself 

 and by other students in laboratory experiments. 



