84 THE ORIGIN OF THE NERVOUS SYSTEM 



are concerned. That is to say, the fully developed 

 organism represents a condition in which most of the 

 irreversible or less readily reversible changes possible in 

 a particular protoplasm under the conditions of life 

 and development have already occurred. It is, in 

 fact, the occurrence of these changes which constitutes 

 development. Under altered conditions reversal, or 

 more properly as regards the process as a whole, regres- 

 sion may occur, at least in the simpler organisms with 

 less stable structural features, and the protoplasm may 

 return to or approach an embryonic or unspecialized 

 condition and undergo development anew (Child, 

 191 56). The earlier stages of development, however, do 

 not and cannot be expected to show the high degree 

 of reversibility of processes that we find in the fully 

 developed organism. Embryonic protoplasm is proto- 

 plasm from which the developmental record has been, 

 so to speak, wholly or in large measure erased, and 

 development is the recording process. 



I have tried to show that the physiological gradient is 

 the first step in the developmental record. If oxida- 

 tion and synthesis are associated, as many authorities 

 now believe, the higher rate of oxidation at the higher 

 levels of the gradient must be a factor in determining a 

 higher rate of synthesis, and more rapid growth may 

 occur at these levels than elsewhere, as is often the case. 

 Moreover, different molecules may be synthesized, or 

 different products of synthesis may be relatively stable 

 and remain in the protoplasm, at the different levels 

 and so differentiation may begin. The chief point for 

 present purposes is that if the conditions giving rise to 

 the gradient persist long enough, whether this be hours, 



