200 



ANIMAL BIOLOGY 



forms these cells are apparently united so that they form nerve 

 nets which surround and permeate the tissues which they stimu- 

 late to action. In more highly developed animals the net arrange- 

 ment is relegated to the control of relatively minor functions, 

 while the main nervous system consists of numerous neurons ar- 

 ranged in groups, or ganglia, and prolongations of the neurons, or 

 nerve fibers, bound together into cables, or nerves. The neurons, 

 which are embedded in protective sheaths of connective tissue in 



"^Dendrites 



Nucleus 



Cell body 



Axon 



Coverings 

 of axon 



Terminal 

 D^ branches 



Fig. 138. — Neurons. Stages in the differentiation of nerve cells. A, prim- 

 itive neuron from the nerve net of Hydra-like animals; B, motor neuron of the 

 Earthworm; C, D, primary motor neurons of a Vertebrate. 



the ganglia, are in physiological continuity one with another by 

 'transmitting contacts,' or synapses; but each neuron, it is be- 

 lieved, remains structurally distinct. (Figs. 138, 139, 143.) 



1. Brain and Spinal Cord 



It will be recalled that the first great differentiation during the 

 development of a multicellular animal establishes an outer ecto- 

 derm and inner endoderm, and thus separates the functions of 

 protection and general reactions to the environment from that of 

 nutrition. It is natural therefore that the ectoderm should be- 



