INTERNAL FACTORS OF REGENERATION IN ANIMALS 5 I 



and this influence may be stronger than any other that might tend 

 to locate the new head in the original middle line. If we suppose 

 that similar conditions prevail in all cases when oblique surfaces are 

 present in these worms, we have a formal solution of the problem. 

 The argument cannot be convincing unless we can give a further 

 explanation of the nature of this influence that the old part has upon 

 the new. 



In other cases, as in the regeneration from an oblique surface in 

 the tail of the tadpole and of a fish, we must assume that the factor 

 that determines the middle of the new part has a stronger influence 

 on the new material than has the most posterior part of the old 

 tissue. 



The influence of an oblique cut-surface on the position of the 

 new parts is shown in a different way in the hydroid, tubularia. 

 The conditions are different in this case inasmuch as there is no 

 proliferation from the cut-end, but the old part produces the new 

 hydranth. Driesch found that if the stem of tubularia is cut in 

 two obliquely, the new tentacles, that develop as two rings around 

 the tube near its cut-end, stand obliquely on the stem, 1 as shown in 

 Fig. 20, A. In most cases, both the distal and the proximal circles 

 of tentacles lie obliquely to the long axis of the stem, but there is 

 some variability in the result, and occasionally one or the other, 

 especially the proximal circle, may be squarely placed, although, as 

 a rule, the influence of the oblique cut-end can be seen. It can be 

 shown, I think, that the oblique position of the rings of tentacles in 

 tubularia is the outcome of factors different from those that are 

 found in the regeneration of the tail of the tadpole and of the head 

 and tail of the planarian. Driesch suggested that the distance of 

 the tentacle-rings from the cut-end is the result of some sort of 

 " regulation " that determines their position at a given distance from 

 the region at which the surrounding water acts on the exposed end. 

 Hence, if the exposed surface is an oblique one the rings will also 

 be formed in an oblique position. On the other hand, I have sug- 

 gested that we can imagine the regulation to result from other 

 factors. At the beginning of the development, and before the 

 tentacles appear, there is a withdrawal of tissue from the cut-end 

 that leaves the region from which the proboscis develops quite thin. 

 If this material withdraws at a uniform rate and to the same distance 

 at all points from the end of the piece, as observation shows to be 

 the case, and if, as appears also to be true, the outer end of the distal 

 ring of tentacles lies at the inner end of the proboscis region, then 

 it too will assume an oblique position if the cut-end is oblique. If 

 we imagine a similar series of regulations taking place throughout 



1 The same holds good for the basal hydranth if it arises near an oblique end. 



