130 



C. M. CHILD 



reaction of this region to the alteried mechanical conditions result- 

 ing from the absence of the posterior end. 



At 10° 44 per cent, twenty-two pieces, were tailless after thirty- 

 six days. These tailless pieces were then brought to a tempera- 

 ture of 20° and left for eight days. At the end of this time ten of 

 the pieces had developed tails pharynges and posterior intestinal 

 branches as indicated in figure 42. The other twelve pieces 

 showed no signs of tail-formation. This result affords a confirma- 

 tion of the results given in table 3 for 20°. At that temperature 

 22 per cent, eleven pieces, remained tailless; at 10° 44 percent, 

 twenty-two pieces remained tailless, butwhen these tailless pieces 

 were raised to 20° only 24 per cent of the total remain tailless. 

 This percentage is almost the same as that of the pieces kept 

 throughout at 20° (22 per cent). 



Figure 42 shows the result of acceleration of the regulatory proc- 

 esses, while the preceding figures show the results of retardation. 

 It is evident from figure 42 that morphogenetic development has 

 occurred to a greater extent at the level of the pharynx than in 

 the postpharyngeal region: in other words, when the rate of reac- 

 tion is accelerated during development the morphological effect 

 is greater or appears more rapidly in the more anterior region. 



In these series we arte concerned only with the pharyngeal and 

 postpharyngeal regions but evidence of the existence of a dynamic 

 gradient appears in these regions in the different effects on mor- 

 phogenesis at different levels of retardation and acceleration of 

 the dynamic processes. 



Series 291. December 21, 1910. From well fed worms 16 to 

 18 mm. in length pieces including the region between the levels 

 2 and 5 in figure 32 were cut. Fifty of these pieces underwent 

 regulation at a temperature of 8° to 10°C. and fifty at 28° to 30°C. 

 The final results, tabulated in percentages are as follows: 



