7o8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



protoplasmic substrate within the range of the transmitted effect may be 

 so altered that a persistent gradient may be established and become an 

 axiate developmental pattern. 



The early differentiation of the central nervous system and the general 

 localization of the chief aggregations of nervous tissue at higher levels, not 

 only of the polar but of the ventrodorsal or dorsiventral gradient, suggest 

 that it represents the most direct physiological and morphological expres- 

 sion in later development of the dynamic factors of earlier stages. More- 

 over, it evidently represents the highest development in each species-proto- 

 plasm of the dynamic integrating factors. This remains true for the verte- 

 brates, even though an inductor is apparently necessary for neural de- 

 velopment. The region known as the "neural inductor" in amphibian de- 

 velopment is a secondary feature of developmental pattern; and direc- 

 tion of its invagination is evidently not independent of the primary pat- 

 tern, nor is the localization of anterior regions of the central nervous sys- 

 tem determined solely by the inductor in development under natural con- 

 ditions. And finally, it is not yet demonstrated that the natural inductor 

 is a substance; but whether it is or not, the presumptive chorda-mesoderm 

 is itself an expression of earlier developmental pattern rather than the 

 basis of pattern. 



The relation to the general gradient pattern of localization of the cen- 

 tral nervous system and of nervous dominance does not exclude the possi- 

 bility that various factors, specific chemical, electrical, or mechan- 

 ical, may be concerned in determining distribution and direction of 

 growth of axons. The suggestion that axons grow up the gradients, den- 

 drites down,^ does not seem entirely in accord with the facts. Certainly 

 in the vertebrate embryo the earlier axons appear to grow down the gra- 

 dients; and in the final pattern of the spinal cord the long axons are those 

 which grow down, while the afferent, upward-growing axons are, in gen- 

 eral, short, as if able to grow only a short distance against some opposing 

 factor. In general, the chief aggregations of nervous tissue are at higher 

 gradient-levels, and the nerves grow down. Local conditions and their 

 changes during development are doubtless concerned in determining the 

 progressive increase in complexity of directions of growth of axons, par- 

 ticularly within the central nervous system. It has been maintained, on 

 the basis of tissue culture experiments, that a physical ultra-structure de- 

 termines the course of growth of nerve fibers (Weiss, 1934)- Whether 

 such structure is the only directive factor is still an open question. 



2 Huxley and De Beer, 1934, p. 380. 



