134 



REGENERATION 



in the new tissue immediately in front of the old part. It lies, 

 therefore, just behind the new head. The proportions of the new 

 worm are at this time very different from those of a typical worm, 

 since the head is much too near to the new pharynx and to the old 

 genital pore. New material is now produced in the region behind 

 the head and in front of the pharynx, so that the head is carried 

 further forward until the new worm has fully assumed the character- 

 istic proportions. As the new head is formed the old part loses its 

 material, so that it becomes flatter and narrower, and if the worm is 

 not fed the old part may lose also something of its former length. 

 If the worm is fed, however, as soon as the pharynx develops the 

 old part loses less and the new part grows forward more rapidly. 



V 



U 



FlG. 41. Posterior end of Planaria lugubris, cut off between pharyngeal and genital pores. 

 Figure to left shows the piece after removal. The four figures to the right show the regenera- 

 tion of the same piece, drawn to scale. As soon as the new pharynx had developed, the 

 worm was fed. The experiment extended from November 17 to January 8. 



The most striking phenomenon in the growth of the new worm is 

 the formation of new material in the region behind the head. The 

 result of this growth is to carry the head forward and produce the 

 characteristic form of the animal. This change is all the more in- 

 teresting since the growth does not take place at a free end, but in 

 the middle of the new material. It is only by the formation of new 

 material in this region that the head is carried to its proportionate dis- 

 tance from the pharynx. It appears that in some way the growth is 

 regulated by influences that determine the form of the new organism. 

 Another experiment on the same animal gives also a somewhat 

 similar result. If a worm is cut in two obliquely (Fig. 21, B) and 

 the regeneration of the posterior piece is followed, it is found that 

 the new material appears at first evenly along the entire cut-surface. 



