54 REGENERA TION 



end of the nervous system is present at a cut-surface, even when such 

 a surface is not at the anterior end of the worm. We may perhaps be 

 able to extend this statement, and state that as many heads will 

 develop as there are exposed anterior ends of the nervous system. 



In two other cases, at least, a somewhat similar conclusion may be 

 drawn, although it appears that in these cases other organs than the 

 nervous system may be the centres around which the new parts 

 develop. Tornier has shown that when the vertebrae of the tail of 

 the lizard are injured, the new material proliferated by the wounded 

 surfaces serve as centres l for the regeneration of new tails ; and 

 Barfurth has found that the notochord in the tail of the tadpole plays 

 a similar r61e in the formation of a new tail. These experiments will 

 be more fully described in connection with the formation of double 

 structures, but from what has been said it will be seen that the cases 

 are parallel to that of the earthworm. 



Until more has been discovered in regard to the internal factors of 

 regeneration, it would be venturesome to make any general statement 

 based on these few cases, but there is opened here a wide field for 

 experimental work. By eliminating one by one the different organs 

 that are present in the old part, it may be possible to discover much 

 more in regard to the internal conditions that are necessary in order 

 that the process of regeneration may take place. 



THE INFLUENCE OF THE AMOUNT OF NEW MATERIAL 



There are certain results connected with the amount of new mate- 

 rial which is produced during regeneration, that should be considered 

 in connection with the question of internal factors. It has been 

 pointed out that when one segment only is removed from the ante- 

 rior end of the earthworm only one new one returns ; when two 

 are cut off two come back, and this holds good up to five segments. 

 Beyond this, no matter how many are removed, only five at most come 

 back. The latter result seems to be connected with the amount of 

 material that is formed over the cut-surface before differentiation 

 begins. When only one or two segments have been cut off, the new 

 material that is formed is soon sufficient in amount for the production 

 of one or two new segments, but when three to five are cut off some- 

 what more material is formed before differentiation begins. When 

 more than five are cut off the new material is at best only sufficient 

 to produce five new ones, and in some cases even a smaller num- 

 ber is formed. This hypothesis assumes that there is a lower limit 

 of size for the formation of new segments below which a segment 



1 Although it is by no means certain that the results may not be due in part, at least, to 

 injuries to the nervous system. 



