PATTERNS OF DEVELOPMENTAL PROGRESS 723 



growth ratio. This law states that the ratio of growth rate of certain 

 parts to growth rate of the whole body is constant for considerable pe- 

 riods.^ The data on which the law is based concern growth in stages 

 following histological differentiation and are chiefly from arthropods and 

 vertebrates; but in these stages the law has been found, according to 

 Huxley (chap, i), to hold for a considerable number of organs of both 

 animals and plants. This growth of parts at a different rate from the 

 whole is heterogenic or allometric growth." It results in a continuous 

 change of proportion of the part to the whole, which may be either posi- 

 tive or negative. However general the validity of the law may prove to 

 be for growth ratios of parts to wholes in later developmental stages, the 

 question of its significance for many growth patterns of embryonic stages 

 remains open. 



GRADIENT PATTERNS AND EVOLUTION OF MORPHOLOGICAL FORM 



Quite apart from the question whether a gradient pattern is the pri- 

 mary pattern of development, it is evident that evolution of morphological 

 form is, to a considerable degree, a matter of change in gradient pattern. 

 Although the characteristics of a gradient pattern depend on the constitu- 

 tion of the protoplasm in which it appears, not on the initiating factor, it 

 and the resulting development can be altered experimentally in a single 

 species-protoplasm by altering physiological condition. The differential 

 modifications of development are cases in point (chaps, v-vii). How ex- 

 treme such modifications may be is indicated by the differential modifica- 

 tions of echinoderm embryonic development (chap. vi). Alteration of form 

 and proportions of the pluteus, exogastrulation, and decrease or oblitera- 

 tion of differences along polar and ventrodorsal axes by differential inhi- 

 bition, changes in form and proportion in the opposite direction by the 

 secondary modifications of differential tolerance, conditioning, or recovery 

 — all these in their various degrees result from differential physiological 

 effects of external agents on gradient patterns. Many of these modifica- 

 tions are highly suggestive as regards evolution of echinoderm larval 

 form. For example, certain degrees of differential inhibition of asteroid 



' The law is expressed by the equation y = bx^, in which x is the magnitude of the animal, 

 as determined by standard linear measurement or by its weight minus the weight of the 

 organ concerned, y is the magnitude of the differentially growing organ, and b and k are con- 

 stants (Huxley, 1932). 



" Originally called "heterogonic" by Huxley, 1932; the term "allometric" was suggested 

 by Huxley and Teissier, 1936. 



