GENERAL PRINCIPLES OF MORPHOGENESIS 113 



and mammals furnish illustrations of the effect of such irreversible 

 differentiation on the course of animal evolution. The significance of 

 this principle in the evolution of the nervous system was briefly dis- 

 cussed in my paper on orthogenesis in liHO, and more fully subse- 

 quently ('21, '22a, '24c, '26, '33rt, '48). 



In the embryo, as soon as connection is made between the sensory 

 zone and the motor zone in the central nervous system, specific 

 peripheral influences begin to operate to a much greater degree, and 

 these may modify the subsequent course of development of the al- 

 ready fabricated sensory and motor apparatus and "inflect" the pat- 

 tern of performance. The nature and degree of this modification by 

 use is, however, limited to the range permitted by the pre-existing 

 intrinsic organization (Coghill, '29, p. 86; Weiss, '41, p. 59). That this 

 range is very restricted in the Amphibia has been made clear by much 

 recent experimental work. The intrinsic growth factors which pre- 

 determine appropriate patterns of nervous organization in embryo- 

 logical development are operative also in the regeneration of periph- 

 eral nerves in the adult animal (see the experiments of Sperry cited 

 on p. 228). 



With progressive increase in the complexity of adjustment to the 

 external and internal environment, there has been a corresponding 

 differentiation of structure. The genetic organization determines the 

 primary pattern, and this pattern is modified by use and the personal 

 experience of the individual. The first of these factors yields that 

 stable organization of tissue which is common to all members of the 

 species and which forms the subject matter of most of the literature 

 of neuroanatomy. The structural changes effected during growth by 

 the second factor are harder to recognize, and this critical field is 

 largely unexplored. In our examination of Ambly stoma we are 

 searching for primordia of both these kinds of tissue — the stable 

 heritable structure and the more labile, individually modifiable tissue 

 involved in conditioning of reflexes and other adjustments to per- 

 sonal experience. Both components are at a low level of differentia- 

 tion in this animal, but their characteristic structure is recognizable, 

 and the successive steps of their further evolutionary development 

 can be followed. 



Most species of urodeles apparently are reversals from more highly 

 differentiated ancestral forms. This, however, in its main features is 

 not a dedifferentiation but an arrest of development, so that adult 

 characteristics of the descendants resemble larval features of their 



