VI.] THE CRANIAL AND SPINAL NERVES. 123- 



About this time, however, the lining of involuted 

 epiblast along the length of the whole spinal cord 

 becomes very much thickened at each side, while 

 increasing but little at the mid-points above and below. 

 The result of this is that the cavity as seen in section 

 (Figs. 64 and 65), instead of being circular, has become 

 a narrow vertical slit, almost completely filled in on 

 each side. 



In the region of the brain the thickening of the 

 lining epiblast follows a somewhat different course. 

 While almost everywhere the sides and floor of the 

 canal are greatly thickened, the roof in the region of 

 the various ventricles, especially of the third and fourth, 

 becomes excessively thin, so as to form a membrane 

 reduced to almost a single layer of cells. (Fig. 40, IV.) 



Cranial and spinal nerves. A most important 

 event which takes place during the second and third 

 days, is the formation of the cranial and spinal nerves. 

 Till within a comparatively recent period embryologists 

 were nearly unanimous in believing that the peripheral 

 nerves originated from the mesoblast at the sides of 

 the brain and spinal cord. This view has now however 

 been definitely disproved, and it has been established 

 that both the cranial and spinal nerves take their origin 

 as outgrowths of the central nervous system. 



The cranial nerves are the first to be developed and 

 arise before the complete closure of the neural groove. 

 They are formed as paired outgrowths of a continuous 

 band known as the neural band, composed of two 

 laminae, which connects the dorsal edges of the incom- 

 pletely closed neural canal with the external epiblast. 

 This mode of development will best be understood by 



