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. 
