Febbuaet 15, 1918] 



SCIENCE 



161 



ondary sense cells, that constitute a taste 

 bud. 



These three types of chemical sense or- 

 gans, genetically related in the order just 

 given, show most interesting physiological 

 differences. Some few substances, like 

 ethyl alcohol, stimulate all three, but at 

 strikingly different concentrations (Parker 

 and Stabler, 1913). If the dilution that 

 will just stimulate the most sensitive of 



-^^ 



Fig. 6. The primary neurones of the three chem- 

 ical senses of vertebrates; A, olfactory; B, com- 

 mon chemical; C, gustatory. In each example the 

 peripheral end is toward the left. 



the three, the olfactory neurones, is ex- 

 pressed as unity, the concentration neces- 

 sary to stimulate the terminals of the com- 

 mon chemical neurones is 80,000 and of 

 the gustatory apparatus 24,000. Hence it 

 appears that when the trophic center is at 

 the receptive end of the neurone, as in the 

 olfactory organ, that end is thousands of 

 times more sensitive than when this center 

 has migrated away from it, though it can 

 recover some of its lost sensitivity by ap- 

 propriating to itself neighboring cells 

 whose nuclear activity may make good in 

 some measure that which was lost by the 

 inward migration of its own trophic cen- 

 ter. Why these centers in the course of 

 phylogeny should have migrated from 

 their original superficial positions inward 

 over much of the length of their neurones 

 is difiScult to say. Possibly it may have 



been due to the advantages of increased 

 nutritive opportunities in these deeper 

 situations or to the establishment of a sec- 

 ond and deeper receptive surface for other 

 .systems of neurones. 



If, as seems to be the case, the proximitj' 

 of the trophic center greatly enhances the 

 sensitivity of the receptive pole of a neu- 

 rone, it is easy to understand why in the 

 differentiation of the protoneurones of the 

 ner\'e-net into the central neurones of the 

 synaptic system the trophic center should 

 migrate toward the receptive pole of the 

 neurone. Such a step is only another as- 

 pect of that whole series of changes that 

 give the synaptic system its high efficiency 

 as compared with that of the nerve-net. 



It might seem at first sight that the mi- 

 grations that have been discussed are the 

 cellular aspects of the general migrations 

 of nerve centers that have been ably and 

 interestingly expounded by Kappers 

 (1907-1917) and his followers, under the 

 head of neurobiotaxis. But these migra- 

 tions, as a moment's reflection will show, 

 are strictly concerned with nervous opera- 

 tions and have to do with the association of 

 groups of neurones in connection with de- 

 veloping reflexes rather than with what 

 may be called the inner life of the neurone. 

 The neurobiotaxes, therefore, are not to be 

 confused with those intraneuronic shifts 

 whereby the trophic center of the nerve 

 cell is placed in such a position as to ad- 

 minister most efficiently to the metabolic 

 needs of the neurone. These shifts give 

 e\adence of the interrelation of the prime 

 factors involved in the organization of 

 every nerve cell, the metabolic and the 

 nervous. Those two factors have been most 

 important in shaping the evolution of this 

 element, but they have not always received 

 at the hands of investigators that separate 

 attention which they deserve. It is one of 

 the objects of this address to emphasize 



