THE BRAIN AND SPINAL COED. 



195 



(2) As the caudal termination of the neural canal is never 

 parated from the epiblast over the coccyx, the posterior end of 

 e cord with its pial covering (mesoblast) is pulled out into a 

 le string — the filum terminate. The neural canal extends for 

 me way into the filum terminale, and in the foetus shows there 

 dilatation or ventricle. The arachnoid and dura mater, meso- 

 astic sheaths of the cord, do not suffer in the retraction of the 

 rd ; they remain widely open to the level of the 2nd sacral 

 rtebra. 



Neurenteric Canal. — The caudal ends of the medullary plates 

 Id over and include within the neural canal the blastopore 

 'ig. 158). The blastopore occurs at the anterior end of the 

 ■imitive streak and marks the point at which the cavity of 

 e hypoblast (that part which becomes the hind-gut) opens 

 i the epiblast. There is thus set up a communication between 

 e neural canal and the gut to which the name of neurenteric 

 ,nal is given (see p. 121). The neurenteric canal also opens 

 to the posterior end of the canal of the notochord. 



Differentiation of the Spinal Part of the Neural Tube. — 

 r hen the medullary plates close in about the 14th day to form 



spongio-bl. 



germ, cells 



post, root gang. 



outer zone 



middle zone 

 inner zone 



ant root 



Fig. 100.— Diagrammatic Section showing the three Zones of the Spinal Neural Tube 

 at the Gth week. 



ie neural tube, they are composed of a layer of elongated epithelial 

 piblastic) cells. By the 6th week these cells have • undergone 

 ie following changes (Fig. 160) : 



