REGENERATION 



165 



d 



the wound itself does not cause head formation at this place. 



It may be assumed that in the differentiation of organs from 

 a regeneration blastema similar mutual influences to those 

 we have encountered in the development of embryos play a 

 role. In fact, the origin of eyes in a regenerate in the flat-worm 

 Polycelis proved to depend on an influence from the cerebral 

 ganglion which spreads over a certain distance (Wolff and 

 Lender, 1950-51). 



The head formed by regeneration at the front edge of a caudal 

 fragment of the body does not at first harmonise with the old 

 tissues of the fragment, 

 because certain intermediate 

 parts of the body and their 

 organs are absent. The 

 normal structure of the 

 organism is restored by a 

 second process, subsequent 

 to that of regeneration in the 

 proper sense. This process is 

 called morphallaxis. In its 

 course, regenerate and frag- 

 ment mutually adjust their 

 sizes (Fig. 56), and, more- 

 over, the newly formed 

 head influences the adjacent 

 tissues in such a way that 

 the missing parts are form- 

 ed here by "reorganisation" 

 of the tissues. If, for ex- 

 ample, the caudal part of a 

 planarian has regenerated a 

 new head, the pharynx which 

 in planarians occupies the 

 middle of the body, at first 

 is absent. By subsequent 

 morphallaxis, however, a 

 cavity is formed in the 

 old tissue of the original 



A 



Fig. 59. Four stages (a, b, c, d) 

 in the regeneration of the rostral 

 end in Sabella. As regeneration 

 proceeds, the foremost 4 segments 

 change from the abdominal into 

 the thoracic type, by the loss of 

 their old hooks and bristles and 

 the formation of new ones in an- 

 other position. This change pro- 

 ceeds in a rostro-caudal succession. 

 After Berrill and Mees. 



