812 THE NERVOUS SYSTEM 



2. The Structural Fundaments of the Nervous System 



The early nervous system shortly after the neural tube is formed is com- 

 posed of an elongated, hollow tube, aggregations of neural crest cells, and a 

 series of sense placodes. 



a. The Elongated Hollow Tube 



The primitive neural tube, located dorsally in the median plane (fig. 217G 

 and H), forms the basis for the central nervous system and potentially is 

 composed of two major regions, namely, the future brain region at its anterior 

 end and posteriorly the rudiment of the spinal cord. The future brain region 

 quickly develops three regions, viz.: 



( 1 ) the prosencephalon, or the rudiment of the forebrain; 



(2) the mesencephalon, or future mid-brain region, and 



(3) the rhombencephalon, or hindbrain region (fig. 354D and E). 



The rhombencephalon passes imperceptibly into the developing spinal cord, 

 or the primitive neural tube posterior to the brain region. 



The cephalic end of the primitive neural tube from the time of its formation 

 tends to present a primary neural flexure, the cephalic flexure (see Chap. 10). 

 This flexure occurs in the region of the mesencephalon. It is slight in teleost 

 fishes, more marked in amphibia, and pronounced in elasmobranch fishes, 

 reptiles, birds and mammals (fig. 354E and F). 



During the early stages of neural tube development, the anterior end of the 

 tube tends to form primitive segments or neuromeres. These neuromeres fuse 

 together as they contribute to the primitive brain regions as indicated in 

 figure 354A-D (see Hill, 1900). 



b. The Neural Crest Cells 



As the neural tube is formed, the neural crest cells come to lie along the 

 dorso-lateral aspect of the neural tube. The crest cells soon become aggregated 

 together in clumps, each aggregation representing the initial stage in the for- 

 mation of the various cranial and spinal ganglia (see figures 347A; 357B-F). 



c. Special Sense Placodes 



The special sense placodes are a series of epithelial thickenings of the lateral 

 portions of the epidermal tube overlying the future head region. These plac- 

 odes, which represent contributions of the epidermal tube to the forming 

 nervous system, are as follows: 



( 1 ) The nasal placodes, two in number, each arising on either side of the 

 ventro-anterior region of the primitive head. 



(2) The lens placodes, two in number, each arising in relation to the 

 optic outpushing of the diencephalic portion of the forebrain. 



