202 THE ORIGIN OF THE NERVOUS SYSTEM 



polarization and of the resulting growth activity is 

 not sufficient to give rise at first to axon structure 

 but gradually becomes so, or else these are cells 

 which react to polarization more slowly than the ordi- 

 nary type of neuron. Bethe has shown that in cer- 

 tain crustacean neurons of this general type (Fig. 52) 

 the physical isolation of dendritic and axonic portions 

 of the cell from the nucleate portions by section of the 

 basal unbranched region of the outgrowths does not 

 interfere with conduction from the dendrites to the 

 axon. Evidently the nucleate portion of the cell has 

 little or no direct relation to the nervous function but 

 is concerned primarily with maintenance. This type of 

 neuron represents then a gradual and partial speciali- 

 zation of the cell body, presumably in the region of high 

 positivity determined by the polarization of the cell. 



THE CELLS OF THE NERVOUS RETICULUM 



These cells (Fig. 66), characteristic not only of the 

 simplest nervous systems in the lower invertebrates 

 but of certain organs in the higher animals, even the 

 vertebrates, apparently have attained little or no 

 definite developmental or morphological polarity. Their 

 branching form and their apparent continuity with 

 other cells may be due, at least in part, as Parker has 

 suggested, to the fact that they represent the persistence 

 of an embryonic syncytium. The absence of a definite 

 structural polarity in these cells may be accounted for 

 either by the indefiniteness and inconstancy of the 

 physiological gradients in the organisms or organs con- 

 cerned and the consequent indefiniteness and incon- 

 stancy of polarization or by the inability of the cells to 



