GRADIENTS, FIELDS, AND DETERMINATION 293 



pattern of molar magnitude in which metabohsm occurs that is determi- 

 native and formative. Properly speaking, formative substances do not 

 exist. Even assuming that localization of a particular substance in rela- 

 tion to the polar pattern of a hydroid determines the level or region where 

 a tentacle shall develop, tentacle development results from a particular 

 spatial activity pattern in that region; this is no less true for the am- 

 phibian limb field. A chemodifferentiation of a field without an activity 

 pattern cannot determine morphological form. There is, at present, no 

 actual evidence that the character of metabolism in the region where a 

 tentacle or a limb will develop is different in earlier stages from that in 

 other regions. Even in later stages the same structures — skin, muscle, 

 connective tissue, bone — develop in the limb, in the tail, and in various 

 other parts of the amphibian body. The difference between these parts 

 is in the spatial developmental pattern rather than in substance. As re- 

 gards anteroposterior and dorsi ventral axes, the amphibian hmb field 

 represents a certain relation to general body pattern, different from that 

 of other parts. In this pattern a new gradient system arises and becomes 

 the longitudinal axis of the limb. The limb as a pattern is specific and 

 undergoes a definite orderly series of changes, with the limb as the re- 

 sult. Different substances are formed in different regions of the pattern, 

 as in other gradient patterns; but again it is the pattern in which the sub- 

 stances are formed, not the substances, which constitutes the limb. 



Determination is commonly supposed to become increasingly stable 

 in the course of development, and this is usually considered to indicate 

 increase in specificity or chemodifferentiation, but determination is some- 

 times apparently merely a matter of gradient-level. A piece of Corymorpha 

 stem from a high gradient-level transplanted to a low level may dominate 

 its environment and develop into an apical region, as it would have done 

 if isolated, and may even induce other parts in the host; a similar piece 

 from a low gradient-level, capable of the same development as the other, 

 when transplanted to a high level may be incorporated and develop 

 merely as part of the stem. The one might be regarded as stably deter- 

 mined, the other as labile. The relative character of lability and stability 

 of determination is shown in many lines of experiment. Various data from 

 isolation and transplantation of parts of the sea-urchin embryo indicate 

 stable determination of the apical region as ectoderm, but in the extreme 

 forms of exogastrulae (Fig. gi,H,I) and with transplantation of micro- 

 meres to the apical pole (pp. 443-44) it is found that the apical region 

 can develop as entoderm. 



