PROCESS OF REGENERATION 101 



forest pools of a characteristic ecological type (see Shelford, '14, 

 pp. 179, 185). This species is active, therefore, only during the 

 spring months when the pools contain water, since it passes 

 into an encysted condition when the water evaporates with the 

 approach of summer. ^ Mrazek ('13) states that such pools form 

 also the typical habitat of Lumbriculus variegatus, and his 

 further remarks on the ecology of this species apply equally 

 well to Lumbriculus inconstans. 



II. THE AXIAL GRADIENT 

 1. The cyanide method 



The most important general fact which has come out of all the 

 work on regeneration is this; — that the result differs according 

 to the level along the antero-posterior axis at which section is 

 made. This difference may be one of rate or of amount of new 

 tissue formed, or, in the most interesting cases, the structure 

 which appears at the cut surface varies according to the level of 

 section. Such an axial difference is obviously the expression of a 

 preexisting internal gradient of some sort, — a protoplasmic 

 gradient independent of the more obvious morphological features 

 of the organism. This gradient must undoubtedly have both 

 structural and functional components, for structure and func- 

 tion always interact; in the living organism they can have no 

 separate existence. But it is the functional component alone 

 with which I propose to deal in this discussion of the axial 

 gradient. 



What means have we for demonstrating a functional, dynamic 

 gradient — or, to put it more simply, a gradient in metabolic 

 processes — along the axis of the living organism? Child ('13 a) 

 has devised and extensively employed a simple method depending 

 on the use of lethal concentrations of anaesthetics and cyanides. 

 In the presence of such substances — ^and of many other poisons 

 also — ^metabolism cannot continue, and if there exists a meta- 

 bolic gradient in the organism, its parts will show a different 



1 A complete account of the life cycle of Lumbriculus inconstans in relation 

 to its peculiar habitat will be published elsewhere. 



