Growth and Regeneration 279 



only the kind of new organ that is produced, but to some extent the 

 rate with which the new part makes its first appearance. For 

 example, in the earthworm a new head will develop only as far 

 back as the 1 5 th to the i8 th segment. Behind that level a reversed 

 tail develops from the anterior end of the piece. The simplest 

 explanation of this is, I think, that the differentiated material 

 of the head has so far diminished when this level is reached, 

 and the differentiated material of a tail is so much in excess, 

 that a tail and not a head is formed. In the region of the 

 15 th to the i8 th segment a head sometimes develops, but it is always 

 imperfect. Conversely, a posterior end will not develop from a 

 posterior cut surface farther forward than about the 1 2 th to the 

 15 th segment. Here, also, we may assume that the head 

 material has so much increased that a new tail no longer de- 

 velops. But this explanation will not account for the different 

 rates of development of a tail at different levels, unless we as- 

 sume that there is more of the tail-differentiated material in the 

 region anterior to the middle of the worm, which is inconsistent 

 with the preceding assumption of the distribution of the materials. 

 Moreover, even if this were the case the assumption would not 

 explain the facts, for a new tail, that is derived from the material 

 at the middle of the worm, also grows more slowly as it reaches 

 the termination of its length. 



Other factors, therefore, must be postulated to account for 

 the difference in the rate of growth at different levels. Two such 

 factors may possibly be recognized. In the first place when a 

 new part is first laid down in the newly proliferated material 

 the terminal part is the first formed, and as much of it is pro- 

 duced as the proportions of the new part allow. For instance, 

 when more than five segments are removed from the anterior 

 end of the earthworm only the five distal segments at most 

 are formed ; and since no growing region is produced between 

 the old and the new part, or in the new part itself (since a 

 growing region is not characteristic of the anterior end), no 

 more than five segments are ever produced. When the pos- 

 terior end of the worm is cut off there is also formed at first the 



