2O4 



BIOLOGICAL LECTURES. 



tion of parts are entirely inadequate at present to make clear 

 the process. More elaborate series of experiments must be 

 undertaken before we can even attempt to make an analysis of 

 these results. 



VIII. 



There are certain problems in connection with the relation 

 between the old parts and the new that need careful study. 

 I have already touched on some of these points in the preceding 

 pages. The limb of a newt, for instance, is replaced by a new 

 one precisely like the old. If only the hand is cut off, just this 

 much is reproduced ; if the forearm and the hand are both cut 

 off, then a new forearm and hand reappear ; and this is true at 

 whatever level the cut is made. In other forms less (or even 

 more) reappears, under certain circumstances, than was removed; 

 but as yet our experiments have not been carried far enough to 

 make clear how this result is related to the renewal of only as 

 much as removed. 



In the earthworm when one, two, three, four, or five anterior 

 segments are cut off, the same number come back ; but when 

 more than five are cut off, only five are regenerated. In 

 LumbriculuSy 1 also, the number of new head segments is never 

 more than seven or eight, even when a much greater number 

 have been cut off. In Planaria a new head forms on the 

 anterior end of a posterior half of the worm, but the entire 

 anterior half is never replaced by new tissue. In this same 

 planarian Randolph has discovered a most important relation 

 existing between the old and new parts. If a planarian be 

 cut in two longitudinally in the median plane, the right half 

 regenerates a new left half of the same size as the part removed, 

 and the left half also develops a new right half of correspond- 

 ing size. If, however, the worm be cut longitudinally into a 

 larger and a smaller strip, the former replaces as much as con- 

 tained in the smaller part that was removed, but the smaller 

 part does not develop the lost larger part, but forms only as 

 much new tissue at its cut side as is about equivalent to its own 

 breadth. 



1 An American species. 



