558 



COMPARATIVE ANATOMY 



Neurolemma cells are added to the nerve either by migration from 

 the neural tube or the dorsal ganglia or from the loose mesenchyma which 

 surrounds the nerve anlage. The migration of medullary cells from the 

 cord appears to be absent in amniote embryos. Some embryologists 

 assume that all neurolemma cells are derived from the dorsal ganglia. 

 The appearance of the medullary sheath is correlated with the enclosure 

 of the neurite by the neurolemma cells. The telodendria of each neurite 

 are associated with the end-plate on the surface of one or more muscle 

 fibers. 



ectoderm 



neural crest 



neural plate 



neural primordium 



crest of dorsal 



Fig. 460. — Drawing showing closure of the neural tube and formation of the neural 

 crest. From pig embryos of: — A, 8 somites; B, 10 somites; C, 1 1 somites; D, 13 somites. 

 X135. (From Patten's "Embryology of the Pig.") 



Visceral motor fibers develop in a manner similar to that of somatic 

 motor fibers, except that the nerve cells are located in the lateral column 

 dorsal to the somatic motor column. Both the nerve cell in the neural 

 tube and the peripheral fiber from it are relatively small and the myelin 

 thin. In the trunk region, the telodendria acquire synaptic connexions 

 with the ganglion cells of the sympathetic ganglia. 



Histogenesis of Sensory Nerves. All sensory neurites develop as 

 outgrowths of cells located in the dorsal ganglia which, in turn, are groups 

 of cells derived from the neural crest. 



When the neural groove closes over and its edges fuse together during 

 the formation of the neural tube, some loose cells are left between skin 

 and neural tube. These ectodermal cells form the neural crest. Whether 

 by this process the neural crest cells are derived from the dorsal wall 

 of the neural tube or from the ectoderm of the skin above matters little, 

 since they are derived, like the neural tube itself, from the ectoderm. 

 The neural crest cells, soon after they first appear, multiply by division. 



