COORDINATION 



201 



come tlie 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 anatomy of the nervous system of Vertebrates, in 

 particular, affords strong evidence of the genetic continuity of 

 the whole series. 



In the development of a Vertebrate, the first indication of the 

 nervous system is a longitudinal groove in the ectoderm along the 

 dorsal surface, which soon becomes converted into a tube by the 



Ventral nerve cord 

 at ganglion 



Motor fiber ending in 

 longitudinal muscle 



Motor neuron cell body 

 Sensory fibers 



Body wall 



Body cavity 



Longitudinal 

 muscle 



Circular 

 muscle 



Epithelium 



Sensory cells 



Fig. 139. — Diagram of primary sensory and motor neurons of the ventral 

 nerve cord of an Earthworm, showing their connections with the skin and the 

 muscles to form a simple reflex arc. See Fig. 61. 



apposition and, finally, the fusion of its edges. This neural tube 

 then becomes separated from, and sinks below the surface ecto- 

 derm, and in time forms the central nervous system consisting 

 of the brain and spinal cord. As development proceeds, out- 

 growths from the central nervous system establish the peripheral 

 and the autonomic nervous systems, so that structurally as well 

 as physiologically the whole nervous system represents a unit; 

 a single organ, as it were, which secondarily becomes closely iden- 

 tified here and there with sense organ, muscle, or gland, as the 

 case may be. (Figs. 142, 174.) 



The first marked structural modifications in the developing 

 central nervous system of Vertebrates are two constrictions of 

 the enlarged anterior end of the neural tube, which establish the 

 three primary brain vesicles: fore-brain, mid-brain, and hind- 

 brain. Thus very early in embryonic development, one end of the 

 neural tube is molded into the brain, leaving the rest to become 

 the spinal cord. (Fig. 140.) 



