CONTROL OF MORPHOGENESIS IN PLANARIA- 321 



The similarity of the first part of this series to the abnormal 

 and cyclopian fish embryos obtained by Stockard is at once 

 apparent. 



III. EXPERIMENTAL CONTROL OF THE CHARATER 

 OF THE ANTERIOR END. 



Attempts to control the appearance of the various types of 

 anterior end were first made in my experiments some five years 

 ago and it was found that various methods of control were possi- 

 ble. The anesthetics were first used for this purpose almost 

 three years ago; a brief report of certain phases of this work has 

 already appeared (Child, 'ioa). Thus far I have been able to 

 control the process of head formation through various internal 

 and external factors. 



In connection with these experiments it has become necessary 

 to standardize the material as far as possible, i. e., to obtain some 

 method of comparing the physiological condition of worms of 

 different size and age, of well fed and starved animals, of ani- 

 mals before and after regulation, pieces from different regions, 

 etc. By the use of alcohol of low concentration (in most cases 

 1.5 per cent.) I have been able to accomplish this to a consider- 

 able extent. A part of the results of this work in their bearing 

 upon the problem of senescence have been described elsewhere 

 (Child, lib). These experiments confirm the conclusion reached 

 from experiments on regulation alone, viz., that in general the 

 capacity for head formation stands in close relation to rate of 

 the metabolic processes in the piece. 



For given conditions a certain rate of reaction is necessary for 

 the formation of a normal head. When the rate falls below this 

 critical level teratophthalmic heads appear first, then as the rate 

 falls still further the teratomorphic and anophthalmic and head- 

 less 'forms appear in succession. Moreover, among the tera- 

 tophthalmic forms the cases of partial fusion represent decrease 

 in rate below the critical level and the single median eyes further 

 decrease. The irregularities of form and position occur most 

 frequently at levels near the old head in small worms and in 

 small pieces from large worms. 



Since length of the piece, region of the body and physiological 



