GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 355 



of the lateral-line, the limbs, ear, gills, and heart, in Amphibia. The 

 lateral-line arises from an epidermal rudiment or placode situated 

 close behind the ear at the early tail-bud stage. It extends down the 

 side of the body by free growth. This can be observed in experi- 

 ments where an anterior half of the body of an embryo of the dark- 

 coloured Rana sylvatica is grafted on to the posterior half of the body 

 of an embryo of the light-coloured Rana palustiis. The lateral-line 

 then grows back as a dark structure on a light background. ^ 



The determination of the path along which the lateral-line will 

 grow is of special interest in connexion with the concept of 

 field-gradient systems. If the tail of a frog embryo is cut off and 

 replaced in an inverted position so that its ventral side is a continua- 

 tion of the dorsal side of the trunk, one of two things may happen. 

 If the tail heals on to the trunk perfectly, the lateral-line will grow 

 back on to the tail and remain at the same level at which it was on 

 the trunk. This means that it grows along a line on the side of the 

 tail, along which it would not have grown if the tail had not been 

 inverted. But if the healing of the tail on to the trunk is imperfect, 

 and the continuity between them is obstructed by scar-tissue, the 

 lateral-line, as it growls back on to the tail, changes its level for 

 the one proper to the tail-region before its inversion (fig. 171). 



It is clear that the track along which the lateral-line grows is not 

 rigidly predetermined, for it can follow a line along w^hich it would 

 normally not have grown. At the same time, the growth of the 

 lateral-line is controlled in relation to the field-system of the body, 

 so that it grows along the antero-posterior axis of the organism at 

 a certain definite level on the dorso-ventral axis. If the inverted 

 tail heals on perfectly, it appears that it comes under the control of 

 the main gradient-system of the embryo, so as to form part of a 

 single unitary field. This will allow the lateral-line to grow back in 

 a straight line without changing its level. If, on the other hand, 

 scar-tissue intervenes between the trunk and the inverted tail, the 

 latter remains in some important way isolated from the main 

 gradient-system of the former, and preserves its old field-organi- 

 sation to which the lateral-line conforms when it comes into its 

 sphere of influence. 



Confirmation of this view is obtained by the experiment of 



^ Harrison, 1904. 



23-2 



