io8 



INDIVIDUALITY IN ORGANISMS 







a 



c 



headless form at the other. I have determined experi- 

 mentally that these different forms 

 represent different degrees of retarda- 

 tion or inhibition of the process of 

 head formation. Their formation can 

 be controlled experimentally in a 

 great variety of ways. For example, 

 the percentage of pieces producing 

 heads, which we may call the head- 

 frequency, is less in shorter than in 

 longer pieces, in pieces from more 

 basal than in those from more apical 

 levels of the body ; less in pieces from 

 young than in pieces from old ani- 

 mals, in pieces from starved than in 

 pieces from well-fed animals, in pieces 

 which are kept quiet than in those 

 forced to move about. 



The effect on head-frequency of 

 substances which decrease metabolic 

 rate, such as dilute solutions of 

 cyanides and narcotics, is of great 

 interest, for it is definite and modi- 

 fiable experimentally, but not uni- 

 form. In series of pieces of equal 

 length, a, 5, c, Fig. 57, taken from 

 animals of the same size and as 

 nearly as possible the same physio- 

 logical condition, the head-frequency 

 under natural conditions is highest in 

 the a-pieces which represent the most 



apical region below the head of the anterior zooid, in 



Fig. 57. — Outline 

 of Planaria doroto- 

 cephala, indicating 

 regions, a, b, c, from 

 which pieces are 

 taken. 



