44 



REGENERATION 



development of the new nerve-cord in lateral pieces, thinks that the 

 new nerve-cord grows backwards in the new part from the brain that 

 develops at the anterior end, either out of the old brain, if it, or any 

 part of it, is left, or out of the new brain that develops from the 

 anterior end of the lateral cord that is present in the piece. What 

 takes place in pieces cut so far to one side that none of the old cord 

 is present in the piece he did not make out ; but I can state that a 



new brain develops even when none 

 of the lateral cord is present. 



The development of a new head 

 in pieces cut to one side of the old 

 median line offers some facts of 

 interest. A piece may be cut from 

 the side of a planarian of such a 

 shape that it has no anterior sur- 

 FIG. 1*9. -indicating how a piece is cut off face at all (Fig. 19, A)j yet a head 



from side of Planar ia maculata. a, 6. develops at the anterior end of the 

 c. Regeneration of last. a. Regenera- .11 i 



tion of single head at side. *. Regenera- new material that appears at the 



side. It stands at first to one side, 



later it assumes an anterior position. In this case an axial structure 

 arises in a lateral position, unless we look upon the new head as 

 arising at the anterior end of the new part, rather than at the side 

 of the old, but there is no evidence in favor of such an interpretation, 

 since the head arises at the same time as does the rest of the new 

 material at the side. In a small piece all of the new material at the 

 side may be used to form the new head (Fig. 19, d}. Sometimes 

 two heads develop (Fig. 19, e). 



REGENERATION FROM AN OBLIQUE SURFACE 



There are also certain important facts connected with the regen- 

 eration from an oblique surface. The first case of the sort was 

 described by Barfurth. He found that if the tail of a tadpole is cut 

 off obliquely, as shown in Fig. 20, B, the new tail that develops stands 

 at first at right angles to the oblique surface. The angle that the 

 new tail makes with the axis of the old tail will be in proportion to 

 the obliquity of the cut-surface. The notochord that occupies the 

 centre of the new tail begins at the end of the old notochord, and 

 extends to the tip of the new tail, dividing it in the same proportion- 

 ate parts as does the notochord of the normal tail. The other organs 

 occupy corresponding positions. As the new tail becomes larger it 

 slowly swings around into line with the old part. This phenomenon 

 of regeneration from an oblique surface has been found in a number 

 of other forms. It has been described by Hescheler, and by myself 



