DYNAMICS OF MORPHOGENESIS 431 



At seventy-two and ninety-six hours the gradient in suscepti- 

 biHty from piece to piece is essentially that of the uninjured ani- 

 mal. At one hundred and twenty hours the same is true, as table 

 12 shows. Here Lot 1 has the greatest susceptibility, Lot 2 is 

 next. Lot 3 next, and Lot 4 sfbout like Lot 3. One-eighth pieces 

 from the posterior region of the first zooid undergo but little re- 

 constitution and almost never produce whole animals while in 

 the more anterior pieces the frequency of complete reconstitution 

 is greater, consequently the differences in susceptibility in table 

 12 one hundred and twenty hours after section are due in part to 

 a greater degree of reconstitution in anterior than in posterior 

 pieces. 



5. The one-twelfth pieces 



The difficulty of cutting even large individuals into as many as 

 twelve pieces of anything like equal size sets a limit to the study 

 of consecutive series of pieces. It is of course possible to cut 

 single pieces much smaller than this with a considerable degree 

 of accuracy but in consecutive series the difficulties are much 

 greater. 



Figure 4 indicates the regions represented by the first six pieces 

 which make up the first zooid. Tables 13 and 14 give the sus- 

 ceptibilities immediately and forty-eight hours after section. 



The susceptibility of all the pieces is very high immediately 

 after section, being higher than that of any of the larger pieces 

 (tables 2, 5, 9) or of the whole worms (table 1). The survival- 

 time of even the least susceptible pieces is only half that of the 

 whole worms and larger pieces. These short pieces are evidently 

 more stimulated by the act of section than are larger pieces. 

 After forty-eight hours, however, (table 14) the susceptibility 

 of all pieces has decreased greatly and is only slightly higher than 

 that of uninjured animals. The effect of section has at least 

 largely disappeared. 



Among the one-twelfth pieces only the more anterior produce 

 whole animals and these do not in all cases. The more posterior 

 pieces usually remain headless. Consequently the later increase 



