58 EARLY AMPHIBIAN DEVELOPMENT 



of which happen to cohere mechanically in a particular form : the 

 only correlations are mechanical ones. 



This lack of co-ordination accounts for the fact that, whether by 

 regulation or regeneration, the making good of material or of parts 

 that have been lost appears to be impossible during this stage of 

 regional self- differentiation of the various organs,^ although regu- 

 lation was possible at the stage of the egg, blastula, and early 

 gastrula, and regeneration will become possible in the larva. The 

 loss of the earlier power of regulation seems to be due to the super- 

 position upon the original unitary gradient-field system of a patch- 

 work of independent chemo-differentiated regions (pp. 221, 350); 

 while the later appearance of the power of regeneration is in the 

 main due to the onset of growth, which in turn depends upon the 

 acquisition of function by the nervous and vascular systems. The 

 latter introduce the possibility of nervous and humoral correlation, 

 and further make possible the mutual interplay of the functions of 

 the various organs as soon as their histological differentiation has 

 proceeded far enough to enable their tissues to function and so 

 permit them to perfect their final development by functional 

 differentiation (see Chap. xiii). 



§9 



From the foregoing sketch it will be obvious that development in 

 Amphibia is epigenetic, and involves the creation of differentiation 

 afresh in each and every generation. There can be no question of 

 preformation, for the structures of the future organism are not 

 there, nor are their positions localised or determined in the un- 

 fertilised egg. This epigenetic character of development is based 

 on the capacity of the protoplasm of the egg to react in a particular 

 way to certain stimuli which in the first instance are external, as 

 when the egg-axis and plane of bilateral symmetry are induced, and 

 then later internal, as when the tissues are induced to differentiate 

 under the influence of an organiser. The whole of development is 

 a series of such reactions or responses to stimuli. It therefore 

 follows that no development can be normal in an abnormal en- 

 vironment, and, also, that the hereditary endowment of an organ- 



^ Harrison, 1915; Spurling, 1923. (See also figs. 22, 94.) 



