34 



REGENERA TION 



mesembryanthemnm, and the piece held so that its basal end comes in 

 contact with a solid body, a root develops at that end. If a piece is 

 held in a similar position, but with its apical end in contact with 

 a solid body, a root does not develop from this end. Evidently the 

 development of a root in this form is also connected with an internal 

 factor ; but that there is in reality a reaction in this case, and not sim- 

 ply the development of a root at the basal end, is shown by the f ollow- 



FlG. 15. After Loeb. A. A piece of the stem of margelis placed in a dish. Roots come off 

 where stem touches dish, and polyps at other points. B. Piece of the stem of tubularia pro- 

 ducing a hydranth at each end. C. Cerianthus membranaceus. Piece cut from side produc- 

 ing tentaeles only on oral side of cut. 



ing experiment : If a piece is cut from the stem and suspended so that 

 both ends are surrounded by water it makes no difference whether 

 the piece is vertical or horizontal a hydranth develops first on the 

 apical end, and then another on the basal end (Fig. 15, B\ When 

 the apical end of a piece is stuck in the sand, leaving the basal end 

 free, a hydranth develops on the latter, but not on the end in the sand. 

 In another hydroid, Margelis carolinensis, studied by Loeb, the 

 effect of contact is more easily demonstrated. If a branch of margelis 



