INTERNAL FACTORS OF REGENERATION IN ANIMALS 



47 



that develops in a piece of this kind appears in the middle of the pos- 

 terior cut-surface, between the old and the new parts. It may extend 

 somewhat obliquely in the new part, and point toward the new tail. 



If a piece is cut from the anterior part of a worm by two oblique 

 and parallel cuts, the new head appears at one side of the anterior 

 cut-surface, and the new tail at the other side of the posterior cut- 

 surface. The new pharynx appears in the new material of the pos- 

 terior part in the middle line. Thus the middle lines of the new 

 head and tail and pharynx lie in different positions, yet these parts are 



FlG. 22. Two upper rows Planar ia lugubris. Lower row Planar ia maculata. Upper row. 

 Tail-piece cut off obliquely in front of genital pore. Figures show mode of regeneration. 

 Middle row. Piece including old pharynx cut off by two cross-cuts, regenerating head and 

 tail. Lower row. Piece cut off as last, regenerating head and tail. 



subsequently brought into the same line. This is done by the head ex- 

 tending more forward and becoming broader, the tail growing backward 

 and also becoming broader. The old piece becomes narrower at the 

 same time. These three changes going on simultaneously produce a 

 new symmetrical worm. In one form, Planaria lugubris, the symmetri- 

 cal form is reached largely by the forward growth and the enlargement 

 of the head, and the growth backward and the enlargement of the 

 tail (Fig. 22, /?). In Planaria maculata the old part shifts, so that it 

 forms a new median line connecting the median line of the new head 



