COORDINATION IN ANIMALS 



185 



developed animals the net arrangement is relegated to the 



control of relatively subsidiary functions (Fig. 103), while 



the main nervous system consists of neurons arranged 



in groups, or GANGLIA, and prolongations of the neurons, 



or nerve FIBERS, bound together 



into cables, or NERVES. The 



neurons, which are imbedded in 



protective sheaths of connective 



tissue in the ganglia, are in 



physiological continuity one with 



another by 'transmitting con- 



tacts/ or SYNAPSES, but each 



neuron, it is believed, preserves 



its structural integrity. (Figs. 



101, 102.) 



It will be recalled that the 

 first great structural differentia- 

 tion during the development of 

 a multicellular animal establishes 



FIG. 102. Diagram of stages in 

 an OUter ectoderm and inner the differentiation of nerve cells 



endoderm, and thus segregates 



the functions Of protection and animals; B, motor neuron of the 



! . . . Earthworm; C, a primary motor 



general reactions tO the environ- neu ron of a Vertebrate. In B and 



from that Of nutrition. It f thenerve impulse passes from be- 



low upward. (After G. H. Parker.) 



is natural therefore that the 



ectoderm should become the seat of those specializations 

 which have evolved into the nervous system and sense 

 organs. Such is the case in all forms from the lowest to 

 the highest and thus the development and comparative an- 

 atomy of the nervous system of Vertebrates, in particular, 

 affords the most cogent evidence of the genetic continuity 

 of the whole series, including Man. 



In the development of a Vertebrate the first evidence of 



