270 OUTLINES OF CHORD ATE DEVELOPMENT 



The elevated medullary folds, or neural folds, soon bend 

 over toward one another and first meet a short distance back 

 from the anterior limit of the head; this is known to be the 

 region of the future mid-brain (Fig. 105). Here the folds soon 

 fuse together forming a superficially continuous sheet of ecto- 

 derm, and an underlying medullary or neural tube. From this 

 point the fusion extends very rapidly posteriorly, very slowly 

 anteriorly. The point where the final closure occurs anteriorly, 

 and consequently the final separation from the superficial ecto- 

 derm, is known as the neuropore; it is the region later distin- 

 guished as the lamina terminalis, and is regarded as the mor- 

 phologically anterior limit of the brain. Topographically, 

 however, this is not the most anterior part of the central nerv- 

 ous system, for during these early stages the rudiment of the 

 brain grows forward and downward, in front of the fore-gut, 

 so that its anterior end becomes bent like a crook. Its floor 

 remains closely applied to the roof of the fore-gut, its extension 

 being due to the growth of the anterior and dorsal regions 

 chiefly (Fig. 99). Thus the morphologically anterior end of the 

 brain really comes to lie on its antero-ventral aspect. 



Certain details in connection with the neural folds and their 

 closure deserve special mention. The actual crests of the 

 neural ridges are flattened, and when the ridges fold together 

 these flattened surfaces form a broad vertical contact. The 

 neural tube is formed by the fusion of the lower or inner 

 margins of these surfaces; the upper margins fuse across the 

 mid-line, restoring a continuous superficial ectoderm. The 

 cells left between the upper and lower margins, derived approxi- 

 mately from the very apices of the neural ridges, are thus left 

 between the neural tube and the surface ectoderm; these are 

 the neural crests (Fig. 102). They do not fuse across the mid- 

 line, but remain as a pair of longitudinal bands along the dorso- 

 lateral surfaces of the neural tube. The neural crests are the 

 rudiments of the ganglia of the cranial and spinal nerves; they 

 are not uniformly developed throughout their extent and may 

 be better marked in some sections than in others. 



The anterior portion of the neural tube expands very con- 



