4 6 



REGENERATION 



cut-end of the old notochord and the median, outer point of the new 

 material, and since the new material is at first equally developed 

 along the cut-edge, or at least symmetrically developed, the new tail 

 must stand at right angles to the cut-edge. This explanation will 

 cover, I think, all cases of regeneration from an oblique surface. It 

 assumes a law of symmetry in the new material that is in accordance 

 with the observed position in which the new structure appears. The 

 hypothesis makes no pretence to explain why the new structures 

 should assume a symmetrical position, but given that they do, the 

 observed result follows. 



There are certain peculiarities connected with the regeneration 

 from an oblique surface in planarians that may be considered in this 

 connection. If the worm is cut in two by means of an oblique cut, 

 as shown by the oblique line in Fig. 21, B, the new head that appears 



\\ I 



\ 



u 



FlG. 21. Planaria lugubris. Upper row. A. Part of head cut off obliquely; a-a*. Regenera- 

 tion of new head. Lower row. B. More of head cut off obliquely ; b-b*. Regeneration of 

 same. 



on the anterior cut-surface of the posterior piece appears at one side 

 and not in the middle of the oblique surface (Fig. 21, B, b}. The new 

 head stands at right angles to the cut-surface. The anterior piece of 

 the worm produces a new tail at the side of the posterior cut-surface, 

 in the same way that the tail is formed in Fig. 20, E. The tail 

 also stands at right angles to the cut-surface. The new pharynx 



