The Behavior of Cells 189 



sorts of functional disturbances would result. What 

 is it that guides the nerve fibers to their proper 

 goal? 



One widely accepted theory of the development 

 of the nerve fiber is th^t it represents an outgrowth 

 from the nerve cell. Harrison by cultivating in 

 hanging drops of lymph small pieces of the nervous 

 tissue from amphibian embryos has been able to 

 observe the actual outgrowth of nerve fibers from 

 the ganglion cells, and to study various stages in 

 the process of outgrowth in living material. Har- 

 rison observed that the extreme tip of the outgrow- 

 ing nerve fiber was often expanded and irregular 

 in contour, and that it underwent amceboid changes 

 as the fiber extended through the lymph. It is not 

 improbable that this amoeboid activity is the cause 

 of the drawing out of the nerve fiber.^ The factors, 

 chemotaxis, thigmbtaxis, or whatever they may be, 

 which direct this amoeboid activity may be respon- 

 sible therefore for the distribution of the nerve 

 fibers in the body and the establishment of the nu- 

 merous connections that occur within the nervous sys- 

 tem itself. The architecture of the nervous sys- 

 tem, the great controlling element in behavior, is 

 not improbably itself a product to a large extent 

 of the peculiar behavior of its component cells. 



1 That the nerve fiber is actually drawn out in the way sug- 

 gested has recently been shown by Mr. J. C. Johnson in his 

 studies of the nerve cells of amphibian larvae. Reference may 

 be made to his paper on "The Cultivation of Tissue of Am- 

 phibians," published in the Univ. of Calif. Publ. Zool. Vol. 16, 



p. ss. 1915- 



