GRADIENT-FIELDS IN POST-EMBRYONIC LIFE 



357 



grafting the anterior half of an embryo of Rana sylvatica into the 

 back of an embryo of Rana paliistris from which the rudiment of 

 the lateral-Hne has been extirpated. The sylvatica embryo has its 

 antero-posterior axis at right angles to that of the paliistris embryo. 

 The lateral-line of the sylvatica head grows back normally under 

 the influence of its own gradient-system, until it reaches the tissues 



Sylvatica 



Palustris 



Fig. 172 

 Effect of the main gradient-field on the direction of growth of the lateral-line. An 

 anterior half-embryo of Rana sylvatica (dark) was grafted on to the back of an 

 embryo of Rana paliistris (light). The sylvatica lateral-line {si), on growing back 

 to reach the paliistris component, bent back to assume the position normal for a 

 lateral-line in the posterior region of the body. (From Harrison, Arch. Mikr. 

 Anat. LXiii, 1904.) 



of the palustris embryo. Here it bends round when it has reached 

 the appropriate level, and continues growing back under the 

 influence of the gradient-field of th^ palustris embryo^ (fig. 172). 



§2 

 The fore-limb rudiment of Amhlystoma at the early tail-bud stage 

 is in the form of a disc of mesodermal tissue at the side of the body 

 (see Chap, vii, p. 222). To each disc there can be ascribed two in- 

 visible axes — the antero-posterior axis, and the dorso-ventral axis, 

 defined relatively to the axes of the whole embryo. If a left limb- 



^ Harrison, 1904. 



