E. NERVOUS SYSTEM. 



THE nervous system, as already mentioned in the Introduction 

 (p. 5), arises from the epiblast, and the first parts to become 

 differentiated histologically are the nerve-cells (ganglion-cells), 

 from which nerve-fibres arise later and serve as conductors of 

 nervous impulses. The most important constituent of the nerve- 

 fibre is a central axis-cylinder or axis-fibre, and in those nerve-fibres 

 which are spoken of as medullated this is surrounded by a highly 

 refractile, fat-like substance (myelin), which forms the medullary 

 sheath. In certain (non-medullated) nerve-fibres this sheath is 

 wanting, but the two kinds of fibres are not sharply marked off 

 from one another, either locally or genetically : a fibre may be 

 medullated in one part of its course, and non-medullated in another. 

 Externally each nerve-fibre is enclosed by a delicate sheath, the 

 neurilemma. 



Part of the epiblastic tissue which forms the nervous system of 

 the embryo does not become transformed into nervous tissue, but 

 gives rise to a supporting and connecting framework the neuroglia\ 

 and externally, investing membranes as well as blood and lymph- 

 vessels, are formed from the mesoblast. 



The nervous system consists of central and peripheral portions 

 (Fig. 121). The central part (brain and spinal cord) is the first 

 to arise, and is formed as a direct product of the epiblast ; the 

 peripheral portion (cerebral, spinal, and sympathetic nerves] becomes 

 established later. 



1. THE CENTRAL NERVOUS SYSTEM. 



The first indication of the central nervous system is a longi- 

 tudinal furrow (medullary groove, Fig. 6, A) which appears on the 

 dorsal side of the embryo and gradually becomes converted into 

 a tube by the meeting of its edges ; this tube, consisting ori- 

 ginally of epithelial cells like the epiblast from which it arises, 

 then becomes separated from the epiblast and gives rise to the 

 hollow medullary cord l (Fig. 6, B), in which nerve-cells and fibres 

 soon become differentiated; it comprises a more expanded an- 

 terior and a longer and more slender posterior section. From the 

 former arises the brain, from the latter the spinal cord. 



1 The cord is at first solid in Cyclostomes, Teleosts, and bony Ganoids, 

 cavity being formed secondarily. 



