FIELDS AND GRADIENTS IN NORMAL ONTOGENY 321 



Obvious examples of the dependent differentiation of a sub- 

 ordinate region under the influence of the dominant (organiser) 

 region interacting with the primary gradient-field are seen in the 

 Amphibia in the formation of secondary embryos after the grafting 

 of an organiser; or the development of engrafted fragments of 

 organs other than the organiser, when the donor has not reached 

 mid-gastrulation, in accordance with their new position instead of 

 their original presumptive fate. The most remarkable of all such 

 cases are the modification of pieces of Anuran presumptive epi- 

 dermis, grafted into the future mouth-region of a Urodele egg, to 



Fig. 149 



Modification of the site of dorsal lip formation in the frog. Left, control egg; 

 right, egg exposed to w/ 10,000 KCN for 24 hours from the 2-cell stage. The 

 dorsal lip {b.p.) is much closer to the equator in the treated egg. (After Bellamy, 

 Biol. Bull. XXXVII, 1919; modified.) 



form a part of the head and jaws which is perfectly organised with 

 the rest of the larva, but which differentiates Anuran structures 

 (e.g. suckers and apparently teeth) never found in Urodeles^ (p. 142). 



(v) The influence of more apical (but not completely apical or 

 dominant) regions on less apical regions 



This is excellently illustrated by the experiments on the 3 2-cell 

 stage in sea-urchin eggs, described in Chaps, v and vi (pp. 1 03 , 1 68), 

 in which it was shown that not only would the basalmost disc of 

 cells (micromeres) induce gastrulation, but so would the sub-basal 

 disc after removal of the micromeres. 



It may also be recalled that the tendency of the animal disc, an. i, 

 is to produce a larva in the middle of which the cilia of the apical 

 organ occupy much too much space and in which no gastrulation 

 takes place, while the tendency of vegetative material is to produce 



^ Spemann, 1932, 1933; Spemann and Schotte, 1932. 



