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SCIENCE 



[N. S. Vol. XL,YU. No. 1207 



ows the final stage of this process as seen 

 in vertebrates where the cell bodies are al- 

 most invariably at the receptive ends of 

 centrally situated neurones (Fig. 4, C). 

 Thus, in the evolution of the protoneurone 

 of the nerve-net into the neurone of the 

 specialized central organ, the cell body mi- 

 grates from a central position to a polar 

 one at the receptive end of the neurone. 



The second type of migration is quite 

 the reverse of that just described. It is 

 seen in the sense cells of the nerve-nets 

 and in those cells that are derived from 

 them and that are associated with the more 

 differentiated sensory surfaces of the 

 higher animals. This type of migration 

 was long ago pointed out by Ketzius 

 (1892) and his account needs only to be 

 supplemented by what is now known of 

 the ccelenterates in order to bring it thor- 

 oughly up to date. In the ccelenterates the 

 sense cell, or receptive protoneurone, has 

 its cell body at its receptive end whence its 

 fibrous prolongation reaches into the 

 nerve-net (Fig. 5, A). Much the same con- 



m 



Fig. 5. Sensory nerve-cells; A, sensory nerve- 

 cell from a ccslenterate ; B, sensory neurone from a 

 mollusk; C, sensory neurone from a vertebrate. 

 In each example the peripheral end of the cell is 

 toward the left. 



dition is found in the earthworm, though 

 in many other worms the cell body has 

 moved to a deeper position, leaving only a 

 process of the cell in connection with the 



sensory surface. In mollusks this inward 

 migration of the cell body is still more pro- 

 nounced (Fig. 5, B.) And finally in ver- 

 tebrates (Fig. 5, C), the cell body of what 

 has now become the primary receptive neu- 

 rone has migrated so far inward as to 

 come to lie much nearer to the central ner- 

 vous organ than to the peripheral receptive 

 surface from which it started. Thus in 

 the two types of cells, peripheral and cen- 

 tral, the directions of migration are oppo- 

 site, for while in the primary sensory neu- 

 rone the cell body has moved away from 

 the receptive pole, in the central neurone 

 it has moved to that pole. 



These migrations, in my opinion, are not 

 to be interpreted as direct expressions of 

 nervous changes in the neurone, as would 

 probably have been surmised by the older 

 school of neurologists. They are the mi- 

 grations of the trophic center of the cell 

 and they probably find their explanation 

 in the changed metabolic needs of the 

 evolving neurone rather than in its im- 

 mediate nervous changes. Something of 

 what these relations are may be gathered 

 from the conditions presented by the recep- 

 tive neurones of the chemical senses of ver- 

 tebrates. Of these the most primitive are 

 the olfactory neurones in which the trophic 

 center is at the receptive end of the cell 

 (Fig. 6, A) reproducing in this respect the 

 conditions found in the integument of sea- 

 anemones and of earthworms. Next in se- 

 quence are the receptive neurones of the 

 common chemical sense (Fig. 6, B) in 

 which the trophic center has migrated far 

 inward toward the central organ, a strictly 

 vertebrate condition. The last members of 

 the series are the receptive neurones of the 

 sense of taste (Fig. 6, C), which are like 

 those of the common chemical sense, except 

 that they have appropriated distally cer- 

 tain integumentary cells, often called sec- 



