ONTOGENY 



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tube, the neural crests are carried up with the neural folds but 

 are not included in the neural tube. After the fusion of the 

 neural folds, the outer layer of ectoderm becomes continuous over 

 the neural tube which is thus cut off from the outside. The 

 closure starts in the region of the future hindbrain and then 

 proceeds forward and back, the anterior end being the last to 

 close. Posteriorly the neural tube connects with the blastopore 

 by the neurenteric canal, which later disappears. The neural 

 crests at first are continuous strands of cells one on each side of 

 the neural tube. Later these strands break up into segments 

 from which develop spinal and parts of cranial nerve ganglia. 



m%. 



m.p. 



Fig. 154. — Semidiagrammatic cross sections illustrating four stages in the 

 development of the neural tube and spinal ganglia of the frog, e, endoderm; g, 

 spinal ganglion; m, mesoderm; m.g., medullary groove; m.p., medullary plate, 

 n, notochord; n.c, neural crest; n.t., neural tube; o.e., outer ectoderm. Inner 

 layer of ectoderm and structures derived from it are shown in solid black. 



The neural tube is the rudiment of both the brain and spinal 

 cord. Sensory axons arising in the neural-crest ganglia grow back 

 to the neural tube to form sensory roots of cranial and spinal 

 nerves. The motor roots of both cranial and spinal nerves 

 originate from nerve cells located in the neural tube. The 

 anterior half of the neural tube becomes constricted into three 

 regions known as the forebrain, midbrain, and hindbrain, which 

 in turn give rise to the five divisions of the adult brain. The 

 ventricles of the brain are derived from the cavity of the neural 

 tube. In the spinal cord this cavity is the central spinal canal. 

 The sympathetic nervous system develops from cells that 

 migrate from the spinal ganglia. Cranial portions of the neural 

 crests in addition to forming ganglionic cells also produce con- 

 siderable quantities of nonnervous mesenchyme which later give 



