126 



C. M. CHILD 



normal animals and sometimes fail entirely to develop a pharynx 

 and posterior end (tailless forms) . Table 3 gives in percentages the 

 results of regulation of such pieces at three different temperatures : 



Each of the three series combined in the table was originally 

 intended for comparison of the effects of different temperatures, 

 but in series 310 and 311 the high temperature lots died because 

 too high a temperature was used: these two series are therefore 

 incomplete but their combination with series 314 gives compara- 

 tive results for three temperatures. 



The percentages of ' tailless' and 'normal' in the table represent 

 final results, i.e., the tailless forms were not cases in which forma- 

 tion of the posterior end was retarded. Very different lengths of 

 time were necessary at the different temperatures; at the highest 

 temperature regulation was complete in about four days, at 20° in 

 eight to nine days and at 10° nearly a month was necessary. The 

 pieces were kept considerably longer than this in order to make 

 sure that tails did not appear later, but as a matter of fact the 

 tailless animal can be readily distinguished from one which is 

 going to form a tail after a relatively early stage by the thickening 

 of the posterior region which appears in the tailless forms. 



The table shows clearly that the percentage of tailless forms 

 increases as the temperature falls. The lower the temperature 

 at which regulation occurs the less frequently do the pieces become 

 wholes. This variation of the regulatory capacity with tempera- 

 ture is apparently due to the fact that at lower temperatures the 

 rate of reaction at the posterior end of the piece is more frequently 

 below the level necessary for tail-formation than at higher. So 

 far the results have no relation to the axial gradient, but when we 

 examine and compare the pieces which have undergone regulation 



