Febbuakt 15, 1918] 



SCIENCE 



159 



ized nerve cells of the synaptic system. 

 Thus we can picture to ourselves the initia- 

 tion of that process which resulted in the 

 production of longer and longer transmis- 

 sion tracts such as we find in the central 

 nervous organs of the higher animals, 

 whereby nerve cells once near neighbors 

 come to be widelj^ separated. In their 

 ontogenetic recovery of connections thus 

 temporarily lost they seem to have failed 

 to reestablish a complete union. This fea- 

 ture of partial recovery, at first a mere inci- 

 dent of growth, contained within it a germ 

 of first importance, for out of it was dif- 

 ferentiated the sjmapse, a device that rein- 

 forced the original polarity of the nerve 

 cell and established a new range of nervous 

 possibilities from which have evolved those 

 highly organized adjustments that make 

 the abode of man's intelligence, his cere- 

 bral cortex, so different from the nerve-net 

 of his digestive tube. 



It will be interesting as new discoveries 

 are made in this field of research to follow 

 in detail the transition from the nerve-net 

 to the synaptic system. At present little 

 is known about this subject, but a very sug- 

 gestive and interesting contribution has 

 been made to it by Moore (1917). It has 

 long been known that strychnine greatly 

 heightens the reflex excitability of many 

 animals and it has been commonly assumed 

 that this action is due to the reduction 

 under the influence of this drug of the 

 synaptic resistances. This being the case 

 strychnine may be used as a test for the 

 presence of synapses. From this stand- 

 point Moore's results are of extreme inter- 

 est, for he has found that the drug has no 

 effect on the neuromuscular responses of 

 coelenterates, a slight one on echinoderms, 

 and a much greater one on crustaceans and 

 moUusks, a series that leads up to the well- 

 known condition in vertebrates and sug- 

 gests in its continuity that the effects are 



dependent upon the appearance and de- 

 grees of differentiation of the sjTiapse. 



Although the nucleated portion of the 

 nerve cell, be it a protoneurone or a neu- 

 rone, is the trophic center and not the ner- 

 vous center of this element, the migrations 

 that this part undergoes in the course of 

 evolution are not without interest. Two 

 lines of movement are observable, one seen 

 in the receptive cells and the other in those 

 of the nerve-net proper. 



Pio. 4. Motor nerve-cells; A, motor cell from 

 the nerve-net of a coelenterate; B, motor neurone 

 from an earthworm; C, motor neurone from a ver- 

 tebrate. In examples B and C the central ends of 

 the cells are toward the left. 



In the primitive protoneurones of the 

 nerve-net in coelenterates (Fig. 4, A) the 

 cell body with its contained nucleus is al- 

 most always centrally located, its processes 

 being in direct connection with those of 

 other like elements. In nerve-nets that 

 exhibit polarization and thus begin to take 

 on the character of differentiated nerve 

 centers, the cell bodies are nearer the re- 

 ceptive than the discharging ends. This is 

 best seen where the process has more nearly 

 reached completion as in the central ner- 

 vous organs of worms, arthropods and mol- 

 lusks (Fig. i, B). Here the cell bodies, 

 usually unipolar, are attached to the trans- 

 mitting axis of the neurones near their re- 

 ceptive poles, and this condition foreshad- 



