198 BIOLOGICAL LECTURES. 



Barfurth, etc.), but the limb does not make a new newt ; 

 because, one may say, it dies before it can bring about the 

 development. But if it were grafted on to the body of the 

 same or of another newt by its distal end, so that its proximal 

 end is exposed, would a new newt bud out from that end ? 

 We should certainly be surprised if this happened. The experi- 

 ment has not been tried with the newt, but a not dissimilar 

 experiment has been made with the rat by Bert. The tail was 

 bent over and fixed beneath the skin of the back of the animal, 

 and after it had grown to that part it was cut off at its root. 

 The exposed end of the transplanted tail did not so far as the 

 experimenter has recorded give rise to a new rat. 1 



The same problem is met with in the regeneration of the 

 earthworm. If a few of the anterior segments (one to five) be 

 cut off, the same number come back ; if more are cut off, the 

 process of regeneration begins only after a longer interval, and 

 only four or five segments come back, as a rule ; if the cut be 

 posterior to the middle, the time before regeneration begins is 

 still longer, and fewer worms succeed in regenerating at all ; 

 and still nearer to the posterior end there is no regeneration 

 anteriorly. In the same worms we find that the posterior end 

 will not regenerate from a small anterior piece, although the 

 piece will regenerate anteriorly if a few of its anterior segments 

 are also cut off. The latter result shows most strikingly that 

 it is not due to the size of the piece that regeneration does not 

 take place posteriorly. We see, therefore, that the posterior 

 end of the body can regenerate in one direction only, namely, 

 posteriorly, and conversely the anterior end can only regenerate 

 anteriorly. There is no structural peculiarity, such as differ- 

 entiation of the cells, that can account for the facts. What, 

 then, is the explanation of this curious phenomenon ? Is there 

 not here a problem that we can hope to solve by means of 

 further experiment? 



1 The experiment is not, perhaps, altogether to the point, since the rat does not 

 regenerate a new tail. 



