310 PRINCIPLES OF EMBRYOLOGY 



Stolon. Most attention has been paid recently to the rate and frequency of 

 the regeneration of the distal end, which gi\es rise to a new hydranth. 

 This is influenced both by the level from which the fragment is taken, that 

 is the distance behind the original hydranth, and by whether a hydranth 

 remains attached to the fragment or not. 



Suppose a piece of Tnhularia or some similar form is taken, consisting 

 of a hydranth attached to an unbranched )it gth of coenosarc. This is cut 

 in two at some level, i.e. at some defmitm^istance not too far behind the 

 hydranth. The usual result is that a new hydranth is formed at the distal 

 end of the proximal piece, but none appears at the proximal end of the 

 distal fragment to which the hydranth is still attached. If, however, in 

 another similar piece, the cut is made very slightly nearer the hydranth, 

 a similar result occurs; and now the hydranth formed on the proximal 

 piece is arising in the very region which, in the previous experiment, 

 failed to form hydranth. Thus this failure must have been due to its having 

 been still in continuity with the original hydranth, and not to any in- 

 herent lack of power of regeneration. This phenomenon, in which the 

 presence of a hydranth suppresses the regeneration of a second hydranth 

 at the other end of a fragment, is spoken of as dominance of the hydranth. 

 (There should be no temptation, of course, to confuse this use of the word 

 with that current in genetics.) (Fig. 14.2.) 



The dominance of an apical hydranth gradually diminishes along the 

 length of the fragment, and at the proximal end of very long fragments is 

 hardly appreciable. This suggests that the hydranth sets up a high level of 

 some activity, or concentration of some substance, which falls off away 

 from it in a gradient along the coenosarc. Rather surprisingly at first 

 sight, there is no evidence of dominance in very short fragments, which 

 tend to regenerate hydranths on the proximal surface as well as the 

 distal. This can, however, fmd an explanation if we suppose that domin- 

 ance only occurs if there is a considerable difference in activity between 

 the two ends of the fragment and that when the isolate is very short, the 

 difference is so small as to be ineffective. 



One camiot accurately measure the degree of dominance in longer 

 pieces without taking into account another factor which varies along the 

 length of the coenosarc. Experiment shows that, quite apart from domin- 

 ance, the inherent rate of regeneration falls off as the distance from the 

 original hydranth increases. Barth (see 1940) investigated tliis by making 

 a series of equal-sized small isolates from the different levels of the stem. 

 Any phenomenon of dominance within the fragments was prevented by 

 constricting them tightly in the middle by a ligature which effectively 

 isolated the two ends from each other. Both ends then produce hydranths. 



