108 C. M. CHILD 



The influence of external factors on regulation or 'experimen- 

 tal reproduction' in Planaria will be considered more fully in 

 later papers. At present we are concerned merely with evidence 

 bearing upon the problem of the axial gradient. The data pre- 

 sented below show the effect of various external factors upon 

 the morphogenic regulatory processes. They comprise a few 

 characteristic series and their number might be greatly increased 

 if it were necessary. But as regards the points to which atten- 

 tion is called these series do not differ in any essential respect 

 from any others recorded in my notes, consequently no useful 

 purpose is served by giving in full the records of large numbers of 

 experiments. Since I have used anesthetics more extensively than 

 other agents and conditions in experiments along the line at pres- 

 ent under consideration, examples of the results obtained with 

 anesthetics are given first and those with other external factors, 

 which supplement and confirm them, follow. 



1. Experiments with anesthetics 



In these experiments the anesthetics were used in such dilution 

 that they did not produce complete narcosis and death but usually 

 allowed regulation to take place to a greater or less extent. The 

 comparison of pieces from different levels and of different regions 

 of the same piece shows, however, that a given concentration of 

 the anesthetic may have a very different effect at different levels 

 along the original axis or the axis of the piece. 



The results obtained in the experiments described below are 

 characteristic and are all confirmed by other series. The anes- 

 thetics were used in various ways : in my earliest experiments the 

 mixture of the desired concentration was placed in a glass dish 

 with ground edge and covered with a glass plate. Every twenty- 

 four or forty-eight hours it was replaced by fresh mixture. Where 

 quantitative results were not required this method was fairly 

 satisfactory. But where different series were to be compared 

 some better method of preventing decrease in concentration was 

 necessary. In many experiments glass dishes with ground edge 

 and fitted cover with ground groove were used. The pieces 



