150 



P. B. SIVICKIS. 



from very short pieces in which a single polarity, in these experi- 

 ments the original polarity, is maintained. They arise when the 

 cells at the posterior cut surface are not sufficiently active in rela- 

 tion to parts anterior to them to grow at the expense of the latter. 

 Heads cut off immediately behind the eyes always remain tailless, 

 and in general when the piece is so short that the posterior cut 

 surface is very close to the new head the development of a new 

 posterior end is inhibited, because the rate of metabolism at other 

 levels of the piece is so high that the cells at the posterior end can 

 obtain but little nutrition. 



Biaxial forms arise in very short pieces of Planaria when a new 

 physiological gradient arises in relation to the posterior cut sur- 

 face. In the short piece conditions are most favorable for the 

 origin of such new gradients because there is but little physiologi- 

 cal difference between the two cut ends — i.e., these short pieces 

 are nearly apolar because they are short, consequently each cut 

 surface may become a dominant region and determine a polarity 

 in the opposite direction to the other (Child, '15&, pp. 98-100). 

 The higher frequency of biaxial heads under ordinary conditions 

 in P. lata than in P. dorotocephala suggests that polarity — i.e., the 

 longitudinal axial physiological gradient — is less stable in the for- 

 mer, and this also is in accord with the conclusion that P. lata is a 

 less specialized form than P. dorotocephala. 



Summary. 



1. Planaria lata possesses a short posterior zooid and undergoes 

 fission. Fission does not interfere with sexual maturity, probably 

 because the level of fission is far posterior to the genital pore. 



2. Susceptibility decreases from the anterior to the posterior 

 end of the first zooid and increases again with approach to the 

 posterior zooid. 



3. Head frequency varies in relation to level of body in the 

 same way as susceptibility. The differences in head frequency at 

 different levels increase as length of piece decreases, and head 

 frequency is slightly lower in young than in old animals. 



4. Isolation of pieces is followed immediately by a great increase 

 in susceptibility, C0 2 production, and oxygen consumption, then a 

 gradual decrease occurs during 12-24 hours to a level still far 



