INTERNAL FACTORS OF REGENERATION IN ANIMALS 49 



in the different parts. In this connection the most important fact is 

 that the growth takes place most rapidly where it will bring about the 

 new form. This problem, which is one of the most fundamental in 

 connection with the phenomena of development and of regeneration, 

 will be more fully discussed in a later chapter. 



A number of assumptions have been made in the above attempt to 

 give an analysis of the formation of a head at the side of an oblique 

 surface. That these assumptions are not entirely arbitrary, but have 

 a certain amount of evidence in their favor, can, I think, be shown. 

 The new material that first appears is supposed to be totipotent, in 

 the sense that any part of it may produce any part of the structure 

 that develops from this material. That this is probable is shown by 

 the following experiment. If a cross-piece is cut from a worm, and 

 then split lengthwise into halves, each half will produce a new head 

 at the anterior edge of the piece. This result shows, at least, that 

 from the tissue lying to the right or to the left of the middle line new 

 material may be formed from which a whole head may develop. The 

 new head does not stand at first with its middle axis in line with the 

 middle of the old piece, i.e. it does not stand squarely at the anterior 

 end of the half-piece, but more towards the inner side of the piece. 

 It may appear that the old part has sufficient influence on the new 

 part to shift the axis of the latter toward the old middle line, but 

 while some such influence may be present, it is probable that the posi- 

 tion of the head is in part the outcome of another factor, viz. the 

 presence at the inner side of the piece of an undeveloped new side, 

 with which the explanation of the less development of the inner side 

 of the head is also connected. 



If a cross-piece is cut from a worm and kept until a small amount 

 of new tissue appears over the anterior and posterior cut-surfaces, 

 and if then the piece is split in two lengthwise, there will develop 

 from each piece a new head out of the new material over the anterior 

 surface. The result shows /\ 



that the new material is at 

 first totipotent, in the sense 



that it may still produce 



one or more heads accord- \J W \ I 



mg tO the Conditions. It ^^_ Planariamaculata . A. Cross-piece, allowed to 

 is possible, of COUrse, that regenerate, then cut in two lengthwise, as indicated by 



.1 r . . r . i line. a-a 5 . Regeneration of left half. 



the formation of the new 



head may have begun at the time of the experiment, but if it had, 

 the development had not gone so far that a new arrangement was 

 impossible. If, however, the piece is not cut lengthwise until just 

 before the formation of a head (Fig. 23, A), then each half-piece pro- 

 duces at first a half-head, that completes itself later at the cut-side. 



