SEGMENTATION 71 



any sense which the word chemical at present connotes. Powers 

 are released by mutilation of which in the normal conditions of 

 life no sign can be detected. All who have tried to analyse the 

 phenomena of regeneration are compelled to have recourse to the 

 metaphor of equilibrium, speaking of the normal body as in a state 

 of strain or tension (Morgan) which when disturbed by muti- 

 lation results in new division and growth. The forces of division 

 are inacessible to ordinary means of stimulation. Applications, 

 for example, of heat or of electricty excite no responses of a 

 positive kind unless the stimuli are so violent as to bring about 

 actual destruction. 6 These agents do not, to use a loose expres- 

 sion, come into touch with the meristic forces. Changes in the 

 chemical environment of cells may, as in the experiments of 

 Loeb and of Stockard produce definite effects, but the facts 

 suggest that these effects are due rather to alterations in the 

 living material than to influence exerted directly on the forces 

 of division themselves. 



By destruction of tissue however the forces both of growth 

 and of division also may often be called into action with a re- 

 sulting regeneration. Interruption of the solid connexion 

 between the parts may produce the same effects, as for example 

 when the new heads or tails grow on the divided edges of Pla- 

 narians (Morgan), or when from each half embryo partially sepa- 

 rated from its normally corresponding half, a new half is formed 

 with a twin monster as the result. 



Often classed with regenerations but in essence quite distinct 

 from them are those special and most interesting examples 

 where the growth of a paired structure is excited by a simple 



over a higher. From the uncertainty whether two given leaves of two separate 

 plants are actually comparable one cannot institute quite satisfactory numerical 

 comparisons, but I think the view that the "Fern" leaf has more lobes than an 

 otherwise similar "Palm" leaf may be fairly maintained. If this be admitted, 

 the "Palm" leaf represents the dominant low number and its round shape is a 

 consequence of the greater powers of growth which are so often possessed by the 

 members of a shorter series. 



6 It is perhaps of importance to remember that in certain species of bacteria 

 (e. g. Bacillus Anthracis) division may cease where the organism is cultivated under 

 certain artificial conditions though growth continues. In this way very long un- 

 segmented threads are produced. 



