62 



REGENERATION 



in all these cases the basal hydranth develops about twenty-four hours 

 after the apical one. In the short pieces, however, the two ends 

 develop at the same time, although the development of all the short 

 pieces, whatever structures they may produce, whether single or 

 double, is delayed, and the hydranths may not appear until after the 

 long pieces have produced their basal hydranths. In these double 



urn 

 Hill 



o 1 



FIG. 30. Tubulariamesembryanthemum. A. Short piece with reduced hydranth-region. B. Piece 

 from distal end of stalk producing a hydranth without a stalk (see Fig. 27, D). C. Piece pro- 

 ducing hydranth as outgrowth of end. C 1 . Later stage of last. D. Short piece producing 

 double proboscis (see Fig. 28, ) . 



structures both ends develop at the same time (Fig. 30, Z>). If we 

 suppose the influences that start the development of the piece begin 

 first at the distal end, the region affected will lie so near to the 

 proximal end of the piece that the development at this end may be 

 hastened, and under these circumstances the region of new forma- 

 tion will be shared by the two hydranths. The factors that deter- 

 mine that a larger, partial structure is formed in preference to a 

 smaller whole one will no doubt be found to be the same in these 

 double structures and in the single ones. 



THE INFLUENCE OF THE OLD PARTS ON THE NEW 



One of the most striking and general facts connected with the phe- 

 nomenon of regeneration is that the new part that is built up on the 

 exposed surface is like the part removed. This suggests that an in- 

 fluence of some sort starts from the old part and changes the part 

 immediately in contact with it into a structure that completes the old 

 part in that region. We can imagine that the new part that has been 

 changed in this way may act on the new part just beyond it, and so 

 step by step the new part may be differentiated. It is not difficult to 

 show that the phenomenon is really more complicated than this, and 

 that other factors are also acting on the new part ; but, nevertheless, 

 that the old part has some such influence is probable. Under certain 

 conditions, however, this influence may be counteracted by other fac- 



