THE NERVOUS SYSTEM 



341 



contain tigroid substance and neurofibrillae characteristic of the nerve 

 cells or neurons of the higher animals. Physiologically, however, they 

 are simpler than neurosensory cells, since they transmit nervous impulses 

 in any direction, while neurosensory cells are definitely polarized and 

 transmit impulses in a single direction only. They are, therefore, inter- 

 preted as neurosensory cells which have lost both their primary connexion 

 with the skin and their functional polarity. (Fig. 303) 



NEUROSENSORY CELL 



BIPOLAR NEURONE 

 ' ' UNIPOLAR NEURONE 



Fig. 302. — A diagram illustrating hypothetical stages in the phylogenesis of the 

 characteristic sensory (afferent) neuron of vertebrates. Earlier stages at the left, final 

 stage at the right. Arrows suggest that the direction of growth of the neurite is away 

 from the source of stimulus. The diagram assumes that the primitive neurosensory cell 

 becomes the definitive sensory neuron and that the definitive sense receptor is secondary. 

 It is quite possible, however, that the neuron is secondary and that the neurosensory 

 cell becomes the definitive receptor. (Redrawn after Ariens-Kappers, modified.) 



In some coelenterates, the primitive ganglion cells form a loose subcu- 

 taneous network or plexus in which, as experiments show, nervous impulses 

 may be carried in any direction. Morphologists incUne to the opinion 

 that the nerve-net of coelenterates becomes the central nervous system of 

 higher animals. 



A final step in the evolution of the neuron is taken by the flatworms 

 in which the neurons, like the neurosensory cells of coelenterates, trans- 



