SUMMARY 441 



morphogenetic agencies come into play. The organism also, through 

 acquiring the power of regeneration, reacquires much of the regu- 

 lative capacity which it lost in its passage through the mosaic stage. 



The type of organisation characteristic of one stage appears to 

 persist, in whole or in part, throughout subsequent stages. Thus, 

 the main gradient-system of the embryo permeates the partial fields 

 of the limb, neural folds, ear, gills, and heart, and determines their 

 axis ; and the growth of the lateral line along a particular level of the 

 flank can best be interpreted in terms of a persistent total gradient- 

 field. 



Again, a total field-system certainly exists in adult Planarians and 

 appears to reveal its presence in late stages of other groups through 

 the presence of growth-gradients permeating the whole organism. 



The persistence into adult life of the partial field- systems of the 

 mosaic stage is shown by the phenomena of regeneration, by the 

 existence of localised growth-gradients within single areas, and 

 notably by phenomena such as those found in newts, where, for 

 instance, indiflferent regeneration-buds produced by an amputated 

 limb will produce legs when grafted into a certain area round the 

 leg, while if grafted near the base of the tail they will produce tails. 



§4 



With this, of course, only a start has been made with the scientific 

 analysis of development. It remains for the future to discover such 

 fundamentals as the physiological basis of the field-systems, and 

 the elaborate physico-chemical processes which must be operative 

 at the time when the quantitative diflFerences of the early gradient- 

 field system are being converted into the qualitative differences of 

 the chemo-diiferentiated mosaic stage. 



It is, however, already a good deal to have arrived at this first 

 outline of development on the biological level. To have established 

 the fact that organisations of quite different type succeed one 

 another during development is important. The recognition of the 

 gradient-field system, with its purely quantitative differentials, as 

 the basis of early organisation, is a great step forward, since it pro- 

 vides an adequate formal explanation of many phenomena of regu- 

 lation which have been considered by various authors, notably by 

 Driesch, as affording proof of vitalistic theories of development. 



