ORIGIN AND DEVELOPMENT OF THE 

 NERVOUS SYSTEM' 



A Review 



Si:\vALL Wrkjht 

 Bureau of Animal Industry, U. S. Department of Agriculture 



THK subject of Professor Child's 

 latest lx)ok is of wider interest 

 than might he thought frtmi the 

 title, dealing as it does with some of the 

 most fundamental problems in biology. 



THREE FUNDAMENTAL PROBLEMS 



The means by which the parts of an 

 organism are coordinated, making it 

 an indi\idual, instead of merely- a mass 

 of li\ing matter, is one of these prob- 

 lems. Three modes of interaction 

 between parts of the body are recog- 

 nized: mechanical stress, transporta- 

 tion of internal secretions in a 

 circulator)- system, and transmission 

 of stimuli directly through the proto- 

 plasm as in the nervous system of 

 animals. In spite of the great empha- 

 sis which recent discoveries have placed 

 on the regulatory action of internal 

 secretions, most biologists will doubt- 

 less agree with Child in consider- 

 ing the nervous system as the most 

 important s(jurce of internal coordina- 

 tion, as it undoubtedly is of external 

 adaptation. 



No less fundamental is the problem 

 as to the agency which controls the 

 course of de\elopment. The orderly 

 series of processes — cell divisions, 

 formation of cell layers, formation of 

 organs by differential growth and 

 complicated foldings, (|ualitati\e differ- 

 entiation intoepitheliuni,ner\-e, muscle, 

 gland, bone, etc., all in projx-r func- 

 tional relations — through which, in 

 the cf)urse of a few weeks, a single, 

 apparently rather simple, microscopi- 

 cal cell converts itself into a highly 

 complex multicellular organism, has 

 been the despair of those who have 

 attempted to explain life phenfmiena 

 in terms of physics iUid chemislr>\ 



DEVELOPMENT IN RESPONSE TO EXTER- 

 NAL STIMULI 



Child emphatically rejects the ex- 

 treme preformist view, under which 

 development consists in the growth 

 and assembling of parts already repre- 

 sented in the egg cell by determiners, 

 and takes place without regard to 

 external conditions so long as the latter 

 are not incompatible with life. He 

 finds the coordinating agency of devel- 

 opment in the same property of living 

 protoplasm of transmitting stimuli 

 which finds its highest development in 

 nerve cells. Development, under this 

 view, is ultimately a reaction to 

 external stimuli. 



The action of a stimulus on the egg 

 cell sets up a temporary excitation- 

 transmission pattern. Excitation means 

 higher metabolism, a higher rate 

 of living. The excitation-transmission 

 pattern thus means a metabolic grad- 

 ient in the egg. Owing to differential 

 behavior of the protoplasm at different 

 rates of metabolism, the prevailing 

 excitation-transmission pattern tends 

 to be accompanied by material differ- 

 entiation, such as holds between the 

 yolk-laden, slowly dividing vegetative 

 pole, and the yolk-free, actively divid- 

 ing animal pole of the egg. This 

 primary "organismic pattern" may 

 have symmetr\- of various sorts; spheri- 

 cal, simple axial, or bilateral, depend- 

 ing on the pattern of action of the 

 external stimuli, which reach the egg. 

 As cell division pnnx'eds, coordination 

 is maintained by continuance of the 

 primary metabolic gradient, now to 

 some extent independent of the con- 

 tinued play of external stimuli, owing 

 to the material differenlation at differ- 

 ent le\els. The gener.d plan of the 



' The Origin and Development of Uu- Nervous Svstem, l)y Cli;irli's .Manning Child, Professor of Zoology 

 University ui Chicago. The University of Chicago Press 192 1. 2% pages with 70 figures. Price SI. 90 

 postpaid. 



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