ORIGINS OF EMBRYONIC PATTERNS 705 



of protoplasmic substances by centrifugal force, and by deformation of 

 the egg (pp. 423-25); but however the polarity is determined, the same 

 sort of individual develops, provided the environment is "normal." In a 

 piece of hydroid stem polarity may be determined by the activation at a 

 cut surface, by electric current, or by an oxygen differential; but the indi- 

 viduals resulting have the same polar pattern, though they may differ in 

 scale of organization. 



Second, developmental pattern is constant and uniform for the species 

 only within a certain range of so-called "normal environment," and even 

 within this range uniformity is by no means complete. Alteration in the 

 essential factors of environment beyond a certain relatively narrow range 

 alters developmental pattern in definite and, to a high degree, predictable 

 ways and may even obliterate it. Differential susceptibility of different 

 gradient-levels may bring about alterations so great that without knowl- 

 edge of the origin of the material its species could not be recognized (see 

 chaps, v-vii). The belief that developmental pattern is highly stable or 

 even autonomous in origin was largely a result of the observational study 

 of embryonic development. With application of experimental methods it 

 became evident that pattern is much less stable and less independent of 

 environment than had been supposed. It is true that the polarity of most 

 animal eggs appears to be relatively stable, but that has been altered 

 experimentally in both invertebrates and vertebrates and even obHter- 

 ated. Probably if earlier stages of ovarian development of the oocyte were 

 accessible to experiment, pattern would be found less stable. Embryonic 

 ventrodorsality and dorsiventrality and asymmetry have been experimen- 

 tally altered, reversed, or obliterated in various species and by various 

 methods.'^'' 



Third, metabolism, of course, requires a substrate; but a local activa- 

 tion and establishment of a gradient or gradient system may occur with- 

 out pre-existing regional differentiation in that substrate, and the metabo- 

 hsm may bring about regional differences. Many cases of reconstitution 

 permit little doubt on this point. The effective factor in development ap- 

 pears beyond question to be metabolism; concentration gradients, mo- 

 lecular arrangements, and morphological pattern are apparently results 

 of earlier metabolic patterns, though all of them, when present, may be- 

 come factors in modifying those patterns. 



19 See chaps, vi, vii, xii, xiii. 



