CHANGE OF SHAPE IN PLANARIA. 28 1 



active, though it never attains anything like normal activity. At 

 the end of twenty or twenty-five days it has acquired a shape 

 like Fig. 3. The posterior end now functions as a tail to some 

 extent and attaches itself to the substratum as the animal 

 advances but the amount of new tissue has not increased. The 

 piece may live for six weeks or more in alcohol but it never 

 undergoes any appreciable further change in shape. The newly 

 formed parts undergo some degree of differentiation but never 

 attain the characteristic adult structure. Apparently the piece 

 has attained approximately a condition of equilibrium for the 

 conditions under which it is living. 



If the concentration of the solution is gradually increased after 

 the first three or four days it is possible to inhibit practically all 

 regulation beyond the closure of the wound : the pharynx does 

 not appear, no further growth of new tissue at the posterior end 

 occurs, and the piece undergoes no elongation (Fig. 4). Under 

 these conditions the piece does not acquire the ability to move 

 about. 



If now these pieces which have attained equilibrium in alcohol 

 be returned to water they gradually resume the process of regu- 

 lation, but with certain differences from pieces which have not 

 been in alcohol. The chief difference for present purposes is 

 that the outgrowth of new tissue at the posterior end does not 

 proceed until it reaches the usual amount. The tail is formed 

 almost entirely by a redifferentiation of the old tissue (Fig. 5). 

 The pieces may undergo change of shape after their return to 

 water until they attain practically the same shape as pieces which 

 have not been in alcohol. Fig. 5 shows a later stage of Fig. 4 after 

 its return to water and Fig. 8 will serve as regards shape for the 

 late stages of either water or alcohol-water pieces. 



These results, which are merely illustrations of what I have 

 observed in several hundred pieces, permit certain conclusions of 

 interest. In the first place it is possible to inhibit entirely the 

 change in shape without inhibiting entirely the processes of redif- 

 ferentiation and regeneration, and the change in shape can be 

 stopped at any point without stopping entirely other regulatory 

 processes (Fig. 3). On the other hand, if the growth of new 

 tissue from the cut surface is inhibited in earlier stages (Figs. 2 



