DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 175 



above the critical level for hydranth development in various hydroid 

 species.' When it falls below that level, stolons develop in some forms; 

 in others there is no development, at least under laboratory conditions, 

 but insufficient food or other incidental conditions may be responsible for 

 its absence. Doubtless stolon metaboHsm is different from hydranth me- 

 tabolism, but apparently a low level of the kind of metabolism which 

 brings about hydranth development makes stolon development possible. 



DIFFERENTIAL MODIFICATION OF PLANARIAN RECONSTITUTION 



Isolated planarian pieces provide interesting and valuable material for 

 the study of differential developmental modification in its relation to de- 

 velopmental pattern. In these pieces it is possible to determine the differ- 

 ential effects not only of external factors but also of certain physiological 

 conditions. Pieces of different lengths from different body-levels, from 

 individuals of different physiological age, nutritive condition, previous 

 conditioning, etc., provide somewhat different starting-points for physio- 

 logical analysis. Moreover, the differential modifications are not limited 

 to the longitudinal axis. 



LONGITUDINAL MODIFICATIONS 



Under natural conditions regeneration of new tissue is more rapid at 

 the anterior than at the posterior end of the piece in Dugesia (see Figs. 

 17, 18), and the amount of new tissue formed in head-development is ap- 

 parently greater than in development of the posterior end. When pieces 

 are subjected immediately after section to concentrations of agents which 

 inhibit reconstitution but are not lethal, development of new tissue is 

 inhibited at both ends, apparently with little, if any, difference. At this 

 time, when activation of the cells near the cut ends is occurring, there is 

 apparently little difference in condition at the two ends. However, if the 

 concentrations of inhibiting agents do not completely inhibit regenera- 

 tion, the anterior new tissue gradually begins to grow and slowly develops 

 into a head, which may be normal or more or less differentially inhibited, 

 while posterior regeneration is almost or quite inhibited. Under these 

 conditions pieces which in normal environment give rise to complete indi- 

 viduals develop into tailless forms, often without pharynx (Fig. 62, A, B). 

 In these more new tissue appears anteriorly than posteriorly, but less 

 than in normal head development; and the head is formed in greater or 

 less part from the old tissue of the piece (Fig. 62, B-D) instead of entirely 



= H. B. Torrey, 1912; Child and Watanabe, 1935^' Barth, 1937^/ J- A. Miller, 1937. 



