SECTION II 

 THE NERVOUS SYSTEM OF VERTEBRATES 



IN these, as in the invertebrata, the central nervous system is developed 

 by an involution of the epiblast, revealing thereby its primitive relations 

 to the surface of the body. At an early period in foetal life, shortly after the 

 formation of the two layers of epiblast and hypoblast, a thickening is ob- 

 served in the epiblast. This thickening soon gives place to a groove, the 

 neural groove (Fig. 140), and the walls of the groove folding over form a 



nuj 



FIG. 140. Transverse section of human embryo of 2'4 mm. to show developing 

 neural canal. (T. H. BRYCE.) 



nc, neural canal 



me, muscleplate ; my, outer wall of somite ; 

 sc, sclerotome. 



canal, the neural canal, which is dilated at the head end of the embryo to 

 form three enlargements known as the cerebral vesicles. 



When first formed the canal is oval in cross-section, its wall being made 

 up of a layer of columnar cells between the outer extremities of which 

 are seen smaller rounded cells. The internal layer of columnar cells send 

 a process peripherally which branches at the end so as to form a close 

 meshwork of fibres. These fibres branch more and more as development 

 progresses, and eventually form the supporting tissue of the adult central 

 nervous tissue, known as the neuroglia. As the wall of the canal grows in 

 thickness, some of the cells may wander outwards and form neuroglia-cells 

 with numerous radiating branches. In the adult nervous system little is 

 left of these cells except their nuclei, so that the neuroglia appears as a close 

 felt- work of fibres, to which here and there nuclei are attached. These cells 



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