156 THE ORIGIN OF THE NERVOUS SYSTEM 



termed the synapse, and a nervous system consisting of 

 distinct neurons connected by means of synapses is 

 known as a synaptic nervous system. In some of the 

 lower invertebrates and in certain organs of the higher 

 animals, however, the nervous system consists of a net- 

 work or reticulum of branching cells forming a syncy- 

 tium, i.e., direct protoplasmic continuity apparently 

 exists from cell to cell through the branches, without 

 the intervention of the membrane characteristic of the 

 synaptic type of connection. The cellular elements of 

 such a nervous network have been called by Parker 

 protoneurons (Fig. 66, p. 203). The protoneurons usu- 

 ally show little or no definite morphological axiation 

 or polarity, but the forms of neuron most characteristic 

 of both invertebrates and vertebrates show very definite 

 axiate patterns, though a wide range of variation in 

 form occurs. 



The problem of the polarity and morphological pat- 

 tern of the neuron is of interest from various points 

 of view : First, as regards its origin, the question whether 

 it is inherent in the neuroblast or is determined in 

 relation to correlative factors is of the same importance 

 as in other cases of axiate pattern. Second, if this 

 polarity is of correlative origin, the question of the 

 nature of the factors concerned is of fundamental 

 significance, since the neuron patterns are essential 

 factors in determining nervous structure and inter- 

 relations. Third, the question of the relation between 

 the visible morphological polarity and the functional 

 polarity of the neuron as an element of the nervous 

 system is an important aspect of the problem of nervous 

 function. 



