7IO PATTERNS AND PROBLEMS OF DEVELOPMENT 



lism has demonstrated their importance in both animals and plants and 

 has also shown that their effects and interrelations become increasingly- 

 definite and complex with increasing differentiation and are therefore most 

 conspicuous and most essential in the higher animals and man. Neverthe- 

 less, since these specific chemical relations are possible only after the parts 

 concerned have become at least chemically, and perhaps morphologically, 

 different, they cannot initiate differentiation or developmental pattern; 

 they are secondary, not primary, factors in development — expressions of 

 pattern already present. They may, of course, play important parts in 

 influencing the further course of development and the character of func- 

 tion. 



INTERRELATIONS OF FACTORS IN INTEGRATION 



Actual physiological control and integration may result from the com- 

 bined action of dynamic and material factors, and the interrelations of the 

 two groups become exceedingly complex and varied in the higher verte- 

 brates and man. It is now a familiar fact that in these organisms hormones 

 may affect the nervous system and alter nervous reactions and general 

 behavior in many ways. Effects of the sex hormones, adrenalin, etc., on 

 behavior are cases in point. And it is no less true that nervous stimuli may 

 influence hormone production or liberation, adrenalin again being an ex- 

 ample. The chemical mediation of nerve impulses by formation or libera- 

 tion at nerve endings of acetylcholine oi sympathin represents a still more 

 intimate relation of dynamic and material factors.-^ Possibly it is not with- 

 out significance in this connection that the vertebrate hypophysis, which 

 appears to exercise, in some measure, a dominance in endocrine relations, 

 is localized, Hke the chief aggregations of central nervous tissue, in the 

 high region of the polar gradient. 



These interrelations, however, have to do with the mature life of higher 

 animals and undoubtedly originate late in development. We have less 

 information concerning interrelations of dynamic and chemical factors in 

 earlier developmental stages, though they may exist after differentiation 

 has taken place. It is reported that presence of the larval cephalic nerv- 

 ous system is necessary in certain insects for metamorphosis from larva 

 to pupa, supposedly because of a substance, a hormone, produced by it.^ 

 The phytohormone, auxin, is generally produced in larger amount in the 

 apical region than elsewhere in the plant axis and is transported more 



•> W. B. Cannon and Rosenblueth, 1937; W. B. Cannon, 1939. 

 ■t Kopec, 1922; Kiihn und Piepho, 1938; Plagge, 1938. 



