RE GENERA TION. 2 O ^ 



place by the development of new tissue, that the new cells have 

 inherited their specific nature from the cells from which they 

 have arisen, meets with no serious objection. But can we 

 explain all the results as the outcome of this inheritance ? I 

 think not. For instance, the assumption will not explain why 

 the new part assumes a form that is often different from that 

 part of the body from which its cells have been derived. For 

 example, if the first five segments be cut from the anterior end 

 of the earthworm, five will come back ; but if more than five 

 are cut off, still only five come back. Now the new cells will 

 in the latter instances be derived from parts of the worm that 

 are quite different in their structure from the fifth or sixth 

 segment, and yet from any level of the anterior end of the body 

 only the five segments develop. The assumption of formative 

 substances, or of a molecular structure in the new cells, might 

 account for the fact that the new ectoderm comes from the old, 

 the new muscles from the old ones, endoderm from endoderm, 

 etc.; but that would leave the main problem unexplained, viz., 

 the development of a new head. This example shows also that 

 Pfliiger's hypothesis is insufficient to account for the result ; 

 for, according to his hypothesis, we should expect that at every 

 level the whole of the missing part should be replaced, and not 

 merely five segments, at whatever level the cut is made. 



We see that other factors are also at work. Let us see if we 

 can account for some of these. The digestive tract, at first end- 

 ing blindly in front, pushes out into the new part until it comes 

 in contact with the ectoderm. Its further growth forward is 

 prevented, not simply by meeting a mechanical obstacle, but 

 by what we may call, for want of a better term, a stimulus 

 received from the point of contact. At this point the ectoderm 

 now turns in to form, as Hescheler has shown, the buccal 

 chamber. We might assume that the endoderm acted in turn 

 as a stimulus on the ectoderm, and the invagination then 

 followed ; but it is probable, from certain results of Rievel 

 and of Hazen, that the invagination would take place, even 

 if the endoderm did not come in contact with the ectoderm. 

 The new nervous system extends forward from the old one. 

 In coming to the anterior end, its further growth forward is 



