ORIGIN OF NEURON PATTERN 



203 



react to such polarization as does occur. Both these 

 factors may be concerned hi many cases. 



Even in the lower invertebrates, however, the 

 reticular nervous structure without definite morpho- 

 logical polarity occurs chiefly in those portions of the 

 nervous system which consti- 

 tute the connecting links be- 

 tween the peripheral receptors 

 and the muscles or other 

 effectors. The peripheral re- 

 ceptors, which are in direct 

 connection with the external 

 surface of the body and there- 

 fore with the physiological 

 factors concerned in surface- 

 interior pattern, do possess a 

 more or less definite polarity, 

 and may give rise to an out- 

 growth which shows definite growth direction with refer- 

 ence to an axial gradient. The receptor cells of the tenta- 

 cles of certain sea anemones, for example, are not only 

 elongated vertically to the surface, but give rise at their 

 inner ends to nerve fibers which grow chiefly down the 

 tentacle, i.e., down the gradient (Fig. 18, p. 112). A 

 more or less definite polarity of this sort is a general 

 characteristic of the peripheral receptors, even in the 

 simplest forms in which we can distinguish definite 

 nerve cells. Even the epithelial muscle cell of Hydra 

 (Fig. 68, p. 238) which, as I have suggested in another 

 chapter (p. 240), is probably to some extent a receptor 

 and conductor as well as a muscle cell, shows a definite 

 polarity with respect to the body surface. Evidently 



Fig. 66. — A cell of the nerv- 

 ous reticulum of a coelenterate; 

 branches of such cells are ap- 

 parently protoplasmically con- 

 tinuous with those of other 

 cells of the reticulum. 



