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 tube (canal) 

 is originally of approximately uniform caliber throughout; but shortly the 



Am. 



Eel 



FIG. 506. TRANSECTION THROUGH THE GUAF 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. 



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.) 



