PATTERNS OF DEVELOPMENTAL PROGRESS 709 



The integrating action of the nervous system in the functional life of 

 higher animals is sufficiently evident from a great variety of experiment 

 and the resulting literature and also from daily life. Moreover, although 

 nervous function is not essential for differentiation of certain parts — for 

 example, vertebrate striated muscle — it is necessary for maintenance of 

 muscle structure after differentiation. Apparently the level of metabo- 

 lism in the developing muscle is high enough for its differentiation without 

 nervous stimuli, but in the differentiated muscle the intrinsic level sooner 

 or later falls below that necessary for maintenance of its structure. Re- 

 constitution and grafting experiments suggest that in planarians, prob- 

 ably in hydroids, and perhaps in annelids, the nervous system is the chief 

 integrating factor in adult life. 



The fact that nervous stimuli do not appear to be important or essen- 

 tial as integrating factors during most of embryonic development may 

 appear, at first glance, to conflict with the view just advanced. Actually, 

 however, nervous function is merely the highest development of a func- 

 tion which we believe to be common to all living protoplasms, that of 

 excitability. Even though transmission of excitation by differentiated 

 nerves plays little or no part in embryonic and some other forms of de- 

 velopment, more primitive transmissions of effects of activations or ex- 

 citations undoubtedly do occur and play an essential part in early physio- 

 logical integration. After the general pattern of an organ system is es- 

 tablished, it may become temporarily more or less independent of pattern 

 in other parts and may undergo more or less self-differentiation, but it is 

 later integrated into the whole, at least in part by nervous factors. 



Production of a specific substance by one part of an organism and its 

 mass transportation to, and action upon, another part is obviously pos- 

 sible only when some degree of difference is present in the parts concerned. 

 As long as they remain qualitatively alike, they produce the same sub- 

 stances, though perhaps at different rates. Differences in concentration 

 of metabolic products at different gradient-levels, resulting from different 

 rates of production, may be concerned in determining further differences; 

 but such effects do not constitute specific chemical relations between 

 parts. Theoretically the specific chemical effect of a product of one part 

 on another becomes possible as soon as specific differentiation of the parts 

 and production of different substances begins. Apparently, however, it 

 does not play a very important part in integration until a considerable 

 degree of differentiation has been attained. The work of recent years in 

 endocrinology and on hormones and other chemical products of metabo- 



