4O REGENERATION 



delay of twenty-four hours in the formation of the reversed head. 

 In Planaria Ingnbris, in which a reversed head develops, if a piece is 

 cut from the anterior end just behind the eyes, the delay in the for- 

 mation of the reversed head is very slight, if indeed there is any 

 delay at all. 



In the earthworm and in the planarian the production of reversed 

 structures appears to be connected with the part of the body through 

 which the cut is made, and to be due to internal factors. The ques- 

 tion arises whether the presence of certain organs at the exposed 

 surface can account for the result. It is conceivable that if such 

 organs are present, and produce new cells that go into the new 

 part, the presence of such cells may be the factor that determines 

 what the new part will become ; and in consequence the polarity 

 of the part may be reversed. For example, the presence of the 

 cut-end of the oesophagus or of the pharynx at the posterior sur- 

 face of the anterior piece of the earthworm may determine that a 

 new pharynx develops at the cut-end, and this may in turn act on 

 the rest of the new tissues in such a way that a head rather than a 

 tail is formed. When a posterior piece is cut off, the presence of the 

 stomach-intestine at the cut-end may influence the new part, so that a 

 tail is produced. It can be shown, however, that a new head may 

 arise at the anterior end of a piece that contains only the stomach- 

 intestine, as sometimes occurs when the worm is cut in two anterior 

 to the middle ; and it is not improbable that a tail can be produced 

 from the posterior end of a piece that contains the old oesophagus, 

 and perhaps even the old pharynx. In the planarian I have espe- 

 cially examined this point, but I have not yet found that the result 

 can be referred to the cut-surface passing through any particular 

 organ, or to the absence of any organs at the cut-end. 



If, instead of referring the result to any one organ, we assume that 

 the tissues near the cut-ends are specialized in such a way that they 

 can only produce their like, and that the sum total of tissues of this 

 sort making up the new part determines the result, we can only sug- 

 gest that this may be so, but we cannot show at present that it is so, 

 or that the result could be brought about in this way. 



We might make an appeal to the hypothesis of formative stuffs, 

 and assume that there are certain substances present in the head, and 

 others in the tail, of such a sort that they determine the kind of dif- 

 ferentiation of the new part ; but this view meets also with serious 

 objections. In the first place, it gives only the appearance of an 

 explanation because it assumes both that such stuffs are present, and 

 that they can produce the kind of result that is to be explained. 

 Until such substances have been found and until it can be shown that 

 this kind of action is possible, the stuff-hypothesis adds nothing to 



