452 Morphogenetic Factors 



that the basis for the norm of the organism is present in the living stuff 

 of every cell. 



The omnipresence of the developmental norm is suggested by that 

 quality in development termed by Driesch equifinality, the attainment of 

 the same developmental goal in very different ways. There is no single 

 or linear progression of steps by which a structure is formed, but the or- 

 ganism may shift its course of development according to circumstances. 

 This is much more difficult to explain than is a linear step-by-step series 

 of changes, each a precursor of the next. A regulatory mechanism of some 

 sort must be involved. What gives unity to the individual is not so much 

 its unchanging genetic constitution, important as this is, but rather this 

 developmental norm, immanent in the organism from the first and often 

 reached over different routes. 



It should be emphasized that what is involved here is not inherent 

 adaptability by which an organism naturally reacts in a favorable way to 

 environmental changes, for a given norm may hinder survival. The "lazy" 

 mutation in maize, for example, which causes the plant to grow flat on 

 the ground, would not persist in nature. Most normative reactions of 

 plants evident today tend to be favorable since they are the ones that 

 have survived in the winnowing process of natural selection, but the fact 

 of normativeness has no relation to adaptability. 



The self-regulatory character of living things has often been observed 

 and discussed. In bodily activities it is the basis of the homeostatic re- 

 actions so evident in physiology. Its most conspicuous manifestations, 

 however, are structural and are seen in those cases where a portion of an 

 organism is experimentally isolated from the rest and then proceeds to 

 restore its missing parts so that a single whole is produced. Each of the 

 first two cells from a Fucus egg, or the first two blastomeres in a frog 

 embryo, if isolated, will develop not into half an organism but into a whole 

 one. Single cells from the epidermis of a plant, under appropriate condi- 

 tions, will develop into whole individuals. Cuttings restore lost roots. 

 Missing shoots are replaced by others. Severed vascular strands are united 

 by the growth of connecting bundles. The literature of plant regeneration 

 is full of such examples. Many of these were brought together by Ungerer 

 ( 1926 ) , but botanists have in general been less concerned with this prob- 

 lem than have zoologists. Regeneration is such a dramatic fact that many 

 attempts have been made to account for it. Actually, however, regenerative 

 development is no more and no less difficult to explain than normal 

 development. The real problem is not regeneration but self-regulatory, 

 normative development. 



Many suggestions have been made to explain this central enigma of 

 morphogenesis, but it must be admitted that none has yet been proposed 

 which is generally acceptable. The problem is enormously difficult and 



