DIFFERENTIAL GROWTH 1 79 



occurs may thereby alter the manifest form which the growth process 

 will take. But it is evident that such actions will be in the nature of 

 merely modifying already established patterns rather than instituting 

 wholly new ones. Consequently an agent applied systemically (non- 

 topically) may accentuate or reduce inherent differentials but cannot 

 create differentials not yet in existence. A few illustrations may be in 

 order. 



As the developmental realization of the primordial growth patterns 

 of the germ progresses, small initial differences among parts become 

 greatly amplified. Minor deviations from normal become exaggerated. 

 Therefore, if each part were to keep on growing independently of the 

 other parts, the harmony of development would soon be endangered. 

 The setting up of new systemic growth controls coordinating the sepa- 

 rate parts thus becomes vital for harmonious development. This is 

 achieved through the humoral, more specially the circulatory, systems 

 for effective and rapid diffusion of substances. But we understand now 

 that a humoral pool could exercise no discriminative control over the 

 various parts supplied by it unless these parts had already previously 

 acquired different response qualities. For instance, the fraction of a 

 common supply that a given part will be able to secure is a function of 

 the accessibility of the part to the circulatory channels; this relation, 

 however, is entirely a matter of previous differentiation and growth of 

 both the part and the vessels. Similarly, the capacity of a part electively 

 to remove, concentrate, utilize, or store certain components from a com- 

 mon pool only reflects affinities of its cells acquired during their prior 

 biochemical specialization; affinity for iodine in the thyroid cell, for 

 vitamin A in the sensory cell of the retina, etc. Or if one cell group re- 

 sponds to a circulating hormone in a manner or degree different from 

 that of another cell group, equally exposed, the two must have been 

 already different constitutionally at the time the hormone appeared on 

 the scene. The case is no different from that of selective drug response 

 of mature cells, which is likewise based on the specific sensitivity of the 

 reacting targets. 



Since no nutrient or hormone or other chemical of ubiquitous distri- 

 bution can make a cell render any contribution for which that cell has 

 not been predisposed by its previous differentiation, none of those agents 

 can be rated as primary causes of differential cell behavior. But they do 

 affect the subsequent elaboration and final proportions of the growth 

 pattern in a variety of ways. A general deficiency of nutrients, oxygen, 

 or specific growth accessories, for instance, hits different growing parts 



