274 



FIELDS AND GRADIENTS 



undetermined as to be able to produce any structures, internal or 

 external. 



On pushing analysis further, it is found that the original control of 

 differentiation in all cases appears to be exerted in relation to what 

 may be called a biological or morphogenetic field. Within these 



Fig. 129 



Gradients of various kinds in the earthworm Pheretima. O O and outer 



left-hand scale, oxidisable substance as determined by the Manoilov reaction. 



X X and outer right-hand scale, solid content, per cent. V V and inner 



left-hand scale, temperature at which heat-shortening occurs. • ©and inner 



right-hand scale, electrical potential, millivolts. (Redrawn after Watanabe, Set. 

 Rep. Tohokii Imp. Univ. vi, 193 1.) 



fields, various processes concerned with morphogenesis appear to 

 be quantitatively graded, so that the most suitable name for them 

 is field-gradient systems, or simply gradient-fields.^ 



^ Historically, concepts of this type were first introduced into embryology by 

 Boveri (1901, 1910) ; later Child (191 5 a) generalised a large number of observa- 

 tions in the form of his theory of axial gradients ; the term field, however, was only 

 introduced in the last few years, notably by Spemann ("Organisationsfeld") 

 (1921), Gurwitsch (1922, 1927), Weiss (1927 c), Bertalanffy (1928), de Beer (1927). 



