160 LIBBIE H. HYMAN 



its duration varying with the different species, and is followed 

 by a depression. 



15. In Lumbriculus inconstans, short pieces from anterior 

 regions are not much stimulated by section, and depression sets 

 in within the time required for head determination. These 

 pieces produce a high percentage of normal heads. Short pos- 

 terior pieces are stimulated much more, and the stimulation 

 lasts for a longer period of time, as long as the time required for 

 head determination; these pieces produce a low percentage of 

 normal heads, and a high percentage of inhibited structures. 



16. The rate of metabolism of the piece during the time when 

 the head is determined is, therefore, the important factor in 

 anterior regeneration in short pieces. If the rate be high, then 

 the region of new tissue which is to form the head is prevented 

 from attaining the degree of independence and isolation neces- 

 sary for normal head formation. Head formation will be inhibited 

 in proportion to the metabolic rate of the old piece. On the 

 other hand, if the metabolic rate of the old piece is low, then the 

 new tissue suffers no inhibition and gives rise to a normal head. 

 The dynamic relations set up between the old and the new^ tissue 

 after cutting determine the cfiaracter of the head, or whether 

 a head shall form at all, or whether a tail shall form. 



17. In long pieces, the dynamic factors are unimportant as 

 the primary gradient determines that the cells at any anterior 

 level are always more independent than those of a more poste- 

 rior level. Therefore, normal heads are always formed on long 

 pieces. 



18. Experimental proof of the above conception is presented. 

 If the rate of metabolism of the piece be depressed by means of 

 cyanide, then the percentage of normal heads is increased; if 

 the rate of the new tissue be decreased, then the percentage of 

 normal heads decreases. 



This work was carried on at the University of Chicago under 

 the direction of Prof. C. M. Child during the years 1911-1914. 

 It is a pleasure to me to acknowledge my indebtedness to Pro- 

 fessor Child, and to express my sincere thanks for his continual 

 kind and helpful criticisms and suggestions, and inspiring com- 



