I2 6 p - B - SIVICKIS. 



develops more rapidly from the median ventral region than from 

 other parts of a cut surface. This fact suggests that internally 

 the median ventral region still possesses the highest metabolic rate. 

 Apparently the primary symmetry gradients undergo more or less 

 alteration in the body wall of some species, and the susceptibility 

 data indicate that such alteration is greater in P. dorotocephala 

 than in P. lata. The latter species seems to retain more nearly 

 the characteristics of earlier stages. 



Young animals are always more susceptible than old and the 

 differences in susceptibility at different levels of the body are less 

 in the young. In fed animals susceptibility decreases from the 

 time of hatching to maturity and in this respect parallels rate of 

 respiration (Child, '19a; Hyman, '19c). In full-grown animals 

 the time from the beginning to the end of disintegration in m/ 1,000 

 KNC at 20° C. is eight to ten hours; in young animals it is much 

 less, increasing with advancing age. High temperature increases, 

 low temperature decreases susceptibility. Starvation increases 

 susceptibility and also decreases the differences at different levels. 

 In all these respects the two species are alike. 



The Forms Resulting from Reconstitution. 



As in P. dorotocephala, the results of reconstitution differ in 

 definite and orderly ways according to length of piece and region 

 of body from which it is taken. Some of these differences, such 

 as the level at which the new pharynx appears, length of pre- 

 pharyngeal and postpharyngeal regions, are merely temporary fea- 

 tures of the earlier stages of reconstitution, and are later largely 

 or completely obliterated by differential growth of prepharyngeal 

 and postpharyngeal regions, particularly if the new individuals are 

 fed, until finally all normal or nearly normal individuals attain 

 approximately the same proportions. 



The most conspicuous differences in the results of reconstitution 

 concern the head and these differences are, with certain rare excep- 

 tions, permanent. Isolated pieces do not always develop anterior 

 ends like that of the normal animal in nature, but abnormal forms 

 occur which constitute a continuous series with some secondary 

 modifications from the normal head to a completely headless con- 



