DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 183 



The physiological factor determining the graded series of head forms 

 in pieces below certain lengths is an effect resulting from section at the 

 posterior end of the piece and the following stimulation of the nervous 

 system and activation of cells there; it is apparently, at least in large 

 part, nervous in character, for section of the nerve cords posterior to the 

 level of head development inhibits the head to almost the same degree as 

 section of the whole body. It is also very clearly shown that the inhibition 

 of the head results from posterior section by delaying either posterior or 

 anterior section for different periods after the other section has been 

 made. Delay of posterior section even for i or 2 hours increases head 

 frequency in short pieces; but with delay of anterior section head develop- 

 ment is inhibited over a much longer period, complete normal head de- 

 velopment being attained only with 3 or 4 days delay. ^ 



Head frequencies are usually lower in pieces from animals kept without 

 food for several weeks before section than in similar pieces from well-fed 

 animals. Probably in the starved animals the cells from which the head 

 develops are less intensely activated and therefore more inhibited by the 

 stimulation resulting from posterior section. In pieces representing equal 

 fractions of body length head frequency is less in those from small, young 

 animals, doubtless because the pieces are shorter and the inhibiting factor 

 from the posterior section therefore more effective than in the longer 

 pieces from large individuals (Child, 191 1/). In general, animals in good 

 condition give a steeper head-frequency gradient than those in poor condi- 

 tion; this gradient is almost invariably steeper in newly collected animals 

 than after long periods in the laboratory (Rulon, 1937). These data sug- 

 gest that head frequency may be altered in two different ways : by alter- 

 ing the condition of the cells concerned in head development cHrectly, and 

 by altering the intensity or effectiveness of the inhibiting factor resulting 

 from posterior section and so indirectly altering the condition of the 

 head-forming cells. The data presented in the following section indicate 

 that both these possibilities are realized with external factors. 



HEAD FREQUENCY IN RELATION TO EXTERNAL FACTORS 



Head frequency may be increased or decreased by environmental fac- 

 tors. With low concentrations of KCN it may be decreased or increased, 

 according to length of piece, body-level of origin, period of exposure, and 

 concentration of cyanide. With the same concentration of cyanide and 



5 For further discussion of the experiments on section of nerve cords and on delay of 

 anterior and posterior section see pp. 406-1 1 ; also Child and Watanabe, 1935a, and Watanabe, 

 1935^- 



