DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 187 



head frequency according to level of origin of the piece (Buchanan, 1922). 

 Long periods of exposure to low concentrations of caffein also decrease 

 head frequency in anterior, and increase it in posterior, pieces, an effect 

 like that of cyanide; but brief exposures to high concentrations mav in- 

 crease the frequency in ante- 

 rior, and decrease it in poste- 

 rior, pieces (Hinrichs, 1924a).'' 

 Induced increase of motor ac- 

 tivity increases head frequen- 

 cy, whether by providing for 

 more adequate respiratory ex- 

 change by change in position 

 or by otherwise stimulating the 

 head-forming cells is uncertain 

 (Child, 1911/). 



Carbon dioxide, hydrogen 

 ion, and certain organic acids 

 are highly effective in increas- 

 ing head frequency in posterior 

 pieces of the anterior zooid but 

 have very little effect in de- 

 creasing it in anterior pieces 

 (Rulon, 1936a, 1937). The ef- 

 fect of CO2 is shown in Figure 

 68. Hydrogen ion in the con- 

 centration found effective in- 

 creases the frequency in the 

 more posterior pieces, where it 

 is low; the increase is much 



B 



C 



D 



E 



Fig. 68. — Increase in head frequencies of 1/8 

 pieces from anterior zooids of 14-16-mm. animals 

 by CO2 {M) over control {L) ; indices as ordinates 

 against body-levels as abscissae (from Rulon, 



1936a). 



greater in pieces from animals previously conditioned to CO, and calci- 

 um antagonizes the effect of hydrogen ion (Fig. 69). 



The action of strychnine (sulphate) is very similar to that of anesthetics 

 and CO2. It increases the frequency greatly in posterior pieces but alters 

 it little or not at all in anterior pieces (Figs. 70, 71) ; it apparently retards 

 development slightly, but the pieces are highly sensitive to stimulation. 



^ In order to show increase in frequency in anterior pieces, it is desirable to use pieces which 

 give relatively low frequencies under standard conditions— for example, pieces from small 

 young animals, from starved animals, or pieces from large animals, but short enough to show 

 a low frequency. 



