588 



THE NERVOUS SYSTEM 



ganglia, and the neurolemma (sheath) cells of both the afferent and efferent 

 fibers of the dorsal and ventral roots respectively. The neural tiibe (canal) 

 is originally of approximately uniform caliber throughout; but shortly the 



Hies. 



FIG. 506. TRANSECTION THROUGH THE GRAF SPEE EMBRYO SHOWN IN THE 



PRECEDING FIGURE. 



Am, amnion; ect, ectoderm; mes, mesoderm; ent, entoderm; M, medullary groove. 

 (From Williams.) 



cephalic end, by a series of processes involving dilatations, constrictions, 

 foldings and fusions, develops into the brain ; while the caudal portion gives 

 rise to the spinal cord. 



... Ganglionic ridge 



Ectoderm 

 Medullary plate 



Ectoderm 



Ganglionic ridge 



Myotome 



FIG. 507. THREE SUCCESSIVE STAGES IN THE PROCESS OF CLOSURE OF THE MED- 

 ULLARY (NEURAL) GROOVE TO FORM THE MEDULLARY (NEURAL) CANAL AND 

 NEURAL (GANGLIONIC) CRESTS. 



(From Barker, after M. von Lenhossek.) 



