Growth and Regeneration 279 
only the kind of new organ that is produced, but tosome 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 15™ to the 18 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"-to the 18" segment a head sometimesdevelops, but it is always 
imperfect. Conversely, a posterior end will not develop from a 
posterior cut surface farther forward than about the 12 to the 
15" 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 
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