PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 145 



earlier in P. lata than in P. dorotocephala. This difference prob- 

 ably accounts, at least in part, for the fact that susceptibility and 

 rate of respiration in pieces of P. lata remain considerably higher 

 after section than in whole animals, while in P. dorotocephala the 

 decrease within twenty-four hours after section is greater, often 

 to the level of whole animals. 



The rate of growth and of differentiation of the head, as deter- 

 mined by the time at which the eyes become visible, is approxi- 

 mately the same in anterior regions and in the posterior zooid in 

 both species, and in both it is more rapid in these regions than at 

 posterior levels of the first zooid. It also decreases and the dif- 

 ferences in rate at different levels increase with decreasing length 

 of piece. In other words, curves of rate of differentiation of 

 head plotted from repeated observations of developing pieces re- 

 semble in their relations to body level and length of piece the 

 head-frequency curves. Comparison of the two species shows, 

 however, that in the shorter pieces and the more posterior levels 

 of the first zooid the rate of differentiation is slightly lower in 

 P. lata than in P. dorotocephala, That is to say, in P. lata the 

 head frequency in such pieces is higher, but the rate of differenti- 

 ation of the head is lower than in P. dorotocephala. 



As regards the portion of body posterior to the head which is 

 formed by new tissue, there is little difference at different levels of 

 section in longer pieces, but with decreasing length of piece this 

 differential appears. In eighths, for example, from levels near 

 the anterior end of the animal and near or in the posterior zooid, 

 the eyes develop at or slightly anterior to the boundary between 

 new and old tissue (Figs. 28, 29), while in pieces from the poste- 



28 & 



Figs. 28-30. Regions of body formed by anterior new tissue at different 

 levels in eighths of P. lata: Fig. 28, anterior region of first zooid; Fig. 29, 

 posterior zooid ; Fig. 30, posterior region of first zooid. 



