NERVOUS SYSTEM 195 



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

 the peripheral portion (cerebral, spinal, and sympathetic nerves, and 

 their ganglia) 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 which gradually becomes converted 

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

 originally of epithelial cells like the ectoderm from which it arises, 

 then becomes separated from the latter, and gives rise to the 

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

 become differentiated ; it comprises a more expanded anterior, 

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

 arises the brain, from the latter the spinal cord. 



In an early stage of development the lumen of the medullary 

 cord is primitively continuous posteriorly with that of the primary 

 intestine by the neurenteric canal, but this connection soon dis- 

 appears. The cord consists of a cylindrical or more or less flattened 

 tube, the cavity of which expands in front to form the ventricles of 

 the brain, and is lined by ciliated epithelium. With the thickening 

 of the walls of the tube, this cavity becomes greatly reduced, and 

 in the spinal cord is spoken of as the central canal. 



Some of the cells in the brain and spinal cord serve as sensory 

 centres, others as motor-centres, new centres being added 

 which complicate the originally simple reflex circuit, and various 

 other modifications gradually occur in the course of development of 

 the head. 



Membranes of the brain and spinal cord (meninges). 



In Amphioxus, the central nervous system is enclosed by an 

 undifferentiated investment of connective tissue. In the lowest 

 Craniata, a differentiation takes place into a primitive meninx, 

 which closely invests the spinal cord, and a second membrane 

 (endorachis), which lines the vertebral canal : the latter, formerly 

 known as the "dura vertebralis," is comparable merely to the 

 perichondrium or periosteum, and has nothing to do with the 

 meninges proper. The blood-vessels supplying the spinal cord are 

 contained in the primitive meninx, the space and tissue directly 

 external to which may be spoken of as the perimeningeal space and 

 tissue. This condition is retained in Fishes (Fig. 146, A). 



A further process of differentiation takes place in Urodeles 

 and is more marked in Anurans, reaching a higher stage in Reptiles 

 and a still higher one in Birds. This process consists in the 



1 The cord is at first solid in certain Fishes (e.g. Petromyzon, Lepidosteus, 

 Amia, Teleostei, Lepidosiren), its cavity appearing later. 



O 2 



