DEVELOPMENT OF THE NERVOUS SYSTEM 473 



at a very early stage of embryonic life ; is, in fact, the first differ- 

 entiation which can be observed after the subdivision of the blas- 

 toderm into its three primary layers. The epiblast very early pre- 

 sents a shallow longitudinal groove bordered on either side by 

 a slight ridge; this is the neural groove, bounded by its neural 

 ridges. 



The ectodermal cells forming the neural ridges multiply much 

 more rapidly than those lying in the bottom of the groove ; conse- 



Headfold 



Neuro- 



pores 



Head plate 



Foregut 



Yolk vei 



Medullary 

 groover-- 



Somite. 



FIG. 373. RECONSTRUCTION OF THE ANTERIOR PORTION OF THE BODY OF A CHICK, THE 



HEAD DISTINCTLY DIFFERENTIATED, SEEN FROM THE SURFACE. (After Kolllliann.) 



quently the ridges come more and more to overhang the groove, 

 and by continued growth they finally meet along the median line. 

 The opposed surfaces of the two lateral ridges then fuse together 

 and the former groove becomes a tube, the neural canal, which is 

 at first much flattened from side to side, its ventro-dorsal being 

 considerably greater than its transverse diameter. At this stage 

 the neural canal resembles a long axial slit bounded upon all sides 

 by epiblastic cells. 



The caudal portion of the neural canal is destined to become 



