I46 P- B. SIVICKIS. 



rior region of the first zooid — i.e., about the level of the pharynx — 

 a considerable portion of the body posterior to the eyes develops 

 from new tissue (Fig. 30). A similar regional differential ap- 

 pears in shorter pieces in P. dorotocephala and P. maculata, but 

 the amount of difference differs somewhat in the different species. 

 In P. foremannii and other species which possess no posterior 

 zooid the regional differential continues to change in the same 

 direction to the posterior end of the body — i.e., the more posterior 

 the piece, the longer the portion of the body formed from new 

 tissue at the anterior end (Morgan, 'oi). 



This regional differential is evidently determined by a complex 

 of factors — e.g., rate of growth of new tissue, size of new head, 

 degree of inhibition in its development by stimulation of the piece, 

 rapidity of reorganization of old parts, and perhaps others. The 

 chief point of interest at present, however, is the fact that this 

 regional differential shows the same relation to body level and 

 length of piece as does head frequency. Its relation to the polar 

 axial gradient is therefore evident. Moreover, experiments show 

 that this differential can be altered and controlled by the same 

 factors by which head frequency is altered and controlled. 



Discussion. 



Re constitution in Relation to Body Level, Length of Piece, and 

 Physiological Age. — It is evident that the axial susceptibility gra- 

 dient is an indicator of fundamental physiological differences along 

 the axis and many facts indicate that such differences are pri- 

 marily quantitative rather than qualitative. Head frequency, dif- 

 ferential increase in susceptibility following section, rate of growth 

 of new tissue at the anterior end, and the portion of the body 

 posterior to the eyes which is formed by new tissue all show a 

 gradation according to body level and therefore a definite relation 

 to the susceptibility gradient. This is true for P.. dorotocephala 

 as well as for P. lata. That the physiological factors which deter- 

 mine these graded differences in reaction to section and in the 

 processes of reconstitution are fundamentally quantitative, not 

 qualitative, is indicated by the fact that there is no evidence of 

 fixity or specificity in their relation to body level. All the features 

 of reconstitution characteristic of a given body level under normal 





