276 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tal pattern with anteroposterior and ventrodorsal axes — or, if the second- 

 ary gradient were in the opposite direction, a dorsi ventral axis — may arise 

 from a pattern consisting of two gradients at right angles, one becoming 

 operative or being much more effective before the other. As a matter of 

 fact, the ventrodorsal or dorsi ventral gradient usually becomes directly 

 evident by the methods at present available, at a later stage of embryonic 

 development than the anteroposterior gradient, although certain experi- 

 mental data indicate that something constituting a basis for it may be 

 present in the unfertilized egg but with little or no visible effect on early 

 developmental stages. As the diagram is drawn, with the high end of the 

 secondary gradient becoming the ventral region, it might be regarded as 

 representing a plane projection of early planarian or annelid pattern. 

 With the high region dorsal, it might perhaps be regarded as resembling 

 the pattern determined in the amphibian Hmb bud by its relations with 

 the pattern of the body. 



In recent years the field concept has been applied to various develop- 

 mental phenomena; but before the word "field" came into use in develop- 

 mental physiology, we find what is essentially the field concept stated 

 in other terms. For example, Spemann (1912a) suggested that early em- 

 bryonic potency to develop an organ such as the lens of the amphibian 

 eye involves an area analogous to a diffraction circle, the degree of deter- 

 mination being highest in the center and decreasing peripherally. Harri- 

 son (19 1 8) says with reference to amphibian limb development: 



The limb rudiment may be thus regarded, not as a definite circumscribed area like 

 a stone in a mosaic, but as a center of differentiation in which the intensity of the 

 process diminishes as the distance from the center increases until it passes away into 

 an indifferent region. Many other systems, such as the nose, ear, hypophysis, gills, 

 seem to have the same indefinite boundaries which may even overlap each other. 



Harrison is speaking here not merely of the limb bud itself, the locus of 

 actual development of the limb or other organ, but of the whole area 

 about this locus, which is found by experiment to be more or less capable 

 of giving rise to the organ. The concept of the developmental or morpho- 

 genetic field has been further developed and applied by various authors 

 independently of, or in relation to, the gradient concept; and, as is usual 

 in such cases, reference of certain developmental activities or results to 

 a field seems sometimes to be regarded as advancing our knowledge of 

 developmental physiology.' Without further analysis the field concept, 



•Spemann, 1921; Gurwitsch, 1922, 1923, 1927; Weiss, 1924, 1926, 1928, 1939; Guyenot et 

 Schotte, 1926a; Guyenot, 1927a, b; De Beer, 1928; von Bertalanffy, 1928; Guyenot et Ponse, 

 1930; Huxley, 1932; Waddington and Schmidt, 1933; Huxley and De Beer, 1934; Dalcq, 1938. 



