PATTERNS AND PROBLEMS OF DEVELOPMENT 



b 



the same exposure period it can be decreased in anterior, and increased 

 in posterior, pieces of the same length from the anterior zooid and again 

 decreased in similar pieces from the posterior-zooid region. In general, in 

 pieces with high head frequency in water these concentrations of cyanide 

 decrease it; in pieces with low frequency they increase it (Child, 1916^). 

 A few data are presented by way of illustration. Table 6 gives head fre- 

 quencies in pieces of different lengths and from different body-levels for 

 controls and different exposure periods with several concentrations of 

 cyanide. In the pieces x of Table 6 and Figure 67, which are so long that 

 the physiological inhibiting factor does not greatly 

 affect head frequency, cyanide decreases it in all 

 concentrations used, and the decrease is greater 

 with continuous exposure to a lower concentration 

 (Table 6, Ix) than after temporary exposure to a 

 higher (Table 6, II:r). In shorter pieces, approxi- 

 mately thirds of the anterior zooid (Fig. 6'j,a,b,c), 

 a-pieces show a decrease in head frequency with all 

 concentrations and exposure periods that are at all 

 effective. Three examples are given in Table 6, la, 

 Ila, and Ilia. In the c-pieces, on the other hand, 

 continuous exposure to low concentrations of cya- 

 nide increases head frequency from the more inhibi- 

 ted forms to teratophthalmic heads (Table 6, Ic) 

 and with shorter exposures may increase frequency 

 of normal heads (Child, 1916^). With higher concen- 

 tration and continuous exposure increase becomes 

 less, is not altered, or decrease occurs. In lie of Table 6 there is an increase 

 from anophthalmic and acephalic forms to teratophthalmic heads, but 

 there is also a decrease from normal to teratophthalmic; consequently, the 

 index shows no significant change. Shorter exposures (48-72 hours) to the 

 same concentration as lie may, however, produce marked increase. But 

 even brief exposures to still higher concentrations decrease frequency in 

 c-pieces, and deaths begin to occur (Table 6, IIIc). The ^-pieces (Fig. 67) 

 which are not included in Table 6 give results intermediate between a 

 and c. Often they show no significant change in frequency under condi- 

 tions in which decrease occurs in a and increase in c. 



Within certain limits high temperature increases, low temperature de- 

 creases, head frequency; pieces from animals conditioned to a certain 

 temperature give different frequencies from those of animals conditioned 



Fig. 67. — Outline in- 

 dicating lengths and lev- 

 els of pieces x, a, and c 

 of Tables 6 and 7 



