H2 THE ORIGIN OF THE NERVOUS SYSTEM 



striking example of such growth of nerve fibers in an 

 axiate organ is found in the tentacles of sea anemones. 

 Groselj (1909) showed that the fibers arising from the 

 sensory cells in actinian tentacles grow predominantly in 

 the basipetal direction (Fig. 18), though in some cells the 

 basal outgrowth divides and gives rise to fibers growing 

 in both directions. Parker (1919, p. 132) concludes that 

 the physiological polarity of such tentacles is dependent 



on this definitely di- 

 rected growth of the 

 fibers. Undoubtedly 

 this conclusion is cor- 

 rect as regards the 

 difference in conduc- 

 tivity in the two di- 

 rections in the fully 

 developed tentacle, 

 but Parker apparently 

 fails to recognize the 

 fact that the directed 

 growth of the nerve 

 fibers is not a mat- 

 ter of independent 

 choice, but must itself be determined by physiological 

 conditions. From the time of their appearance as 

 buds, these tentacles show a definite physiological gra- 

 dient, with differences in electrical potential, suscepti- 

 bility, reducing power, etc., and the directed growth of 

 the nerve fibers is unquestionably determined by this 

 pre-existing gradient. 



While the evidence is by no means complete, all 

 that we have indicates very clearly that the differentia- 



Fig. 18. — Sensory cells of the sea- 

 anemone tentacle and their outgrowths; 

 the arrow points toward the base of the 

 tentacle (from GroSelj, 1909). 



