168 REGENERATION 



that were too small did not produce complete polyps on a 

 smaller scale, but only parts of hydranths of normal size. 

 On this point, therefore, the field may behave in either of two 

 different ways. As yet it is entirely unknown which factors are 

 responsible for the choice between the two. 



Further, it was found that the area eventually occupied by 

 the field depends upon the "state of activity" of the regenerate 

 acting as organiser. This, in turn, can be influenced by external 

 factors. If, under the influence of cold or narcotics, the re- 

 generated head of a planarian has remained small, the pharynx 

 which arises by morphallaxis under the influence of the field 

 also remains smaller, and it is formed at a shorter distance 

 from the rostral end. There is a direct correlation between the 

 size of the head, that of the pharynx, and the distance between 

 the two (Child, 1915), (Fig. 58). Similar phenomena have been 

 found in hydroids. In Sabella, illumination of the regenerating 

 head causes a strong expansion of the field. In darkness, only 

 about four abdominal segments develop into thoracic segments 

 during morphallaxis, but this number may be increased to as 

 many as eighty by illumination (Berrill and Mees, 1936). 



The organising power proved not to be a privilege of the 

 extreme rostral, or apical, part of the body. As a rule, each 

 part of the body has an organising influence on the more caudal 

 parts. A fragment from the middle of the body of a planarian 

 can regenerate a tail at its hind edge, even if no regeneration 

 of a head takes place at its rostral end. Even a pharynx may 

 be formed in this case, at least if the fragment originated from 

 the pre-pharyngeal part of the body, but not if it originated 

 from the post-pharyngeal region. This agrees with the observa- 

 tion by Okada and Sugino (1934) that not only transplanted 

 heads, but also more caudal parts of the body of a planarian 

 have inductive effects v/hen grafted into still more posterior 

 regions of another individual. 



The same applies to the polyp Corymorpha. Here parts of 

 the stem, grafted laterally into the stem of another polyp, in a 

 number of cases induce a new hydranth. This happens more 

 often, according as the original position of the graft in the 

 stem was more apical (Child, 1929), (Fig. 61). 



