THE ECTODERM AND ITS DERIVATIVES 185 



by the 9 mm. stage from the fourth cranial crest segment and 

 placode and becomes associated with the neuroblasts from 

 the medulla. Peripherally the branches feed the second 

 and third branchial arches, the tympanum, the muscles of 

 the shoulders, the viscera (including larynx, oesophagus, 

 stomach, lungs, and heart), and the lateral line system. The 

 non-nervous portion of the crest segment forms mesenchyme 

 and the superficial non-nervous elements of the placode 

 then disappear. 



Summary of Derivatives of the Cranial Nerves in Tabular Form 



Cranial Crest Segment Cranial Nerves Derived Therefrom 



( V ophthalmic branch 

 V mandibular branch 

 II VII and VIII 



III IX 



IV X 



The Spinal Nerves 



The spinal nerves, aside from their component parts, differ from 

 the cranial nerves in that they are related to mesodermal somites rather 

 than visceral clefts. 



The spinal nerves arise from the pair of continuous neural crests. 

 These are elongated strands of neural ectoderm left outside of the 

 dorsal neural tube, between it and the dorsal ectoderm, at the time of 

 closure of the neural tube. These crests become metamerically seg- 

 mented and are paired as a result of the development of the somites. In 

 frontal sections of the 7 mm. stage these crests will be seen as dorsal 

 root ganglia of the spinal nerves. No placodes are developed in asso- 

 ciation with any part of the neural crests, as were described in associa- 

 tion with some of the homologous cranial crests. 



Each crest segment is made up of many neuroblasts which send 

 fibers to the dorso-lateral wall of the spinal cord to form the dorsal 

 root or ramus of the spinal nerve. This connects the dorsal root 

 ganglion (original neural crest) with the spinal cord. Also, other fibers 

 grow ventro-laterally from this ganglionic crest to become the afferent 

 tracts from the skin and other sensory organs. 



Within the spinal cord, as early as the 4 mm. stage, neuroblasts de- 

 velop and send out bundles of efferent axons (fibers) which emerge 

 from the ventro-lateral wall of the spinal cord and almost immedi- 



