PHYSIOLOGICAL REGENERATION 1 29 



each moult the body increases in size ; but after the pupa stage is 

 passed and the imago formed, there is no further moulting. In the 

 Crustacea, on the other hand, the adult animals moult from time to 

 time, and the upper limit of size is less well denned than in the insects. 

 The larvae also pass through a series of moults. 



An interesting case of physiological regeneration has been de- 

 scribed by Balbiani in a unicellular form, stentor. From time to time 

 a new peristome appears along the side, moves forward and replaces 

 the old peristome, that is absorbed as the new one comes into position. 

 In other infusoria the peristome may be absorbed before encystment, 

 and a new one appear when the animal emerges from the cyst. 

 Schuberg states that when division takes place in bursaria the new 

 peristome develops on the aboral piece in the same way as after 

 encystment ; and Gruber observed that, when an aboral piece of an 

 infusorian is cut off, a new peristome develops in the same way as 

 after normal division of the animal. These observations indicate that 

 the process of physiological regeneration may follow the same course 

 and probably involves the same factors as the process of restorative 

 regeneration. 



Tubularia absorbs its old hydranth-heads if placed in an aquarium, 

 and regenerates new ones. It may even absorb the hydranth while 

 growing in an aquarium, as Dalyell has shown, and presumably, there- 

 fore, also under natural conditions. After each regeneration the new 

 stalk behind the head increases in length. 



In plants, in which there is a continuous apical growth, new parts 

 are being always added at the end of the stem, and old parts are con- 

 tinually dying, as seen in palms. Most trees and shrubs in temperate 

 climates lose their leaves once a year and produce new ones in the 

 spring. Since the new leaves develop from the new shoots at the end 

 of the stem and branches, the old ones can, only in a general way, be 

 said to be renewed. 



That a very close relation exists between the process of physio- 

 logical regeneration and restorative regeneration will be sufficiently 

 evident from the preceding illustrations. We do not gain any insight 

 into either of the processes, so far as I can see, by deriving the one 

 from the other, for the process of restorative regeneration may be, in 

 point of time, as old as that of physiological regeneration. This does 

 not mean, of course, that the same factors may not be present in both 

 cases. So similar are the two processes that several naturalists have 

 attempted to show how the process of restorative regeneration has 

 been derived from physiological regeneration. Barfurth, recognizing 

 the resemblance between the two processes, speaks of restorative 

 regeneration as a modification of physiological regeneration, and 

 Weismann also supports this point of view. He says : " Physiological 



