192 Regeneration 



remarked incidentally that such eggs at the time of cell 

 division contained the centrosomes and astrospheres, 

 and yet went back into a resting state^ thus showing 

 that the centrosomes are only transitory organs or 

 organs which are only active under certain conditions. 

 It is quite possible that in these phenomena of reversal 

 not the whole of the cortical layer has undergone altera- 

 tion. 



The writer must leave it undecided whether the 

 changes from the resting to the active state in body 

 cells can also be explained in analogy with these experi- 

 ments. 



9. In the formation of the lens we have already 

 noticed an instance where the adjacent organ influences 

 growth inasmuch as the optic cup controlled the for- 

 mation of the lens. Such influences are quite commonly 

 observed. A piece of Tuhularia when cut out from a 

 stem and suspended in water will regenerate at the 

 aboral pole not a stolon but a polyp, so that we have an 

 animal terminating at both ends of its body in a head. 

 The writer called such cases in which an organ is 

 replaced by an organ of a different kind hetero- 

 morphosis. 



Contact with a solid body favours the formation of 

 stolons. Fig. 36 shows a piece of a stem of Pennaria 

 another hydroid, which was lying on the bottom of an 

 aquarium and which formed stolons at both ends a and 

 b. In Margelis, another hydroid, the writer observed 



