GENERAL PRINCIPLES OF MORPHOGENESIS 111 



purposes of descriptive anatomy and experimental physiology an 

 analysis in terms of functional efficiency is indispensable, and a mor- 

 phology which ignores the dynamic factors of tissue differentiation in 

 terms of physiological adaptiveness lacks something and is sterile, as 

 I have repeatedly emphasized ('08, '10, '13, '226, '25a, '33a). 



This does not imply that in either embryonic or phylogenetic de- 

 velopment the pattern of structural differentiation is determined pri- 

 marily by peripheral influences, which, acting directly on the germ- 

 plasm, produce heritable changes in its structure. Whether such in- 

 heritance of acquired characters ever occurs is in controversy. Cer- 

 tainly it is the general rule that adaptation of structure to physio- 

 logical requirements is effected by indirection — natural selection or 

 some other principle — but that it is acquired in some way is evident, 

 and these physiological requirements are key factors in morphogene- 

 sis. This is more evident in the nervous system, perhaps, than in 

 other organs of the body, for one of the prime functions of nervous 

 tissue is adjustment of the body to its environment. 



The analysis of morphogenic factors is best accomplished experi- 

 mentally, as is well illustrated by Holtfreter's recent study ('45). 

 This paper sets out in sharp relief the contrast between growth and 

 differentiation. The relation between intrinsic and extrinsic factors 

 and between nuclear and cytoplasmic influences in morphogenesis is 

 discussed in two recent books by W. E. Agar ('43, chap, v) and R. S. 

 Lillie ('45, chap. x). The reciprocal relations between genes and 

 cytoplasm are now under active investigation. 



There is decisive evidence that in embryogenesis the pattern of 

 differentiation of both sensory and motor systems is determined 

 largely by intrinsic agencies and that it proceeds more or less au- 

 tonomously up to functional capacity. These structural patterns are 

 laid down in the inherited organization, and that organization has 

 been elaborated in the course of phylogenetic development in adapta- 

 tion to the physiological needs. of the species in question. Morphology 

 here ties in with ecology. 



The last point carries with it the necessary implication that the 

 intrinsic agencies which initiate and shape the course of embryonic 

 differentiation are not strictly autonomous and that even the heredi- 

 tary factors have arisen ab initio as responses of the protoplasmic 

 organization to environmental influences. It is this point of view 

 which has been emphasized by Child ('41) in his search for more 



