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B 



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L 



S 





FIG. 1. Diagrammatic illustration of the emergence of regional differences from 

 a morphologically uniform system in the spinal motor column of the chick 

 embryo, (a) 3-day embryo: The motor column is of uniform width from the 

 cervical to the sacral level, (b) 5-day embryo: The majority of the differ- 

 entiating neuroblasts in the cervical segment of the motor column undergo de- 

 generation. They are represented as solid black circles. In the thoracic and sacral 

 segments the migration of the preganglionic columns is under way. (c) 8-day 

 embryo: The degenerated nerve cells in the cervical segment of the motor column 

 have disappeared. The remaining nerve cells in this segment form the slender 

 medial motor columns. Note the size increase of the brachial and lumbar motor 

 columns innervating the limbs. In the thoracic and sacral segments, the two pre- 

 ganglionic columns have reached their definitive position adjacent to the central 

 canal. The two slender columns in a peripheral position represent the medial motor 

 columns. C, cervical level; B, brachial level; L, lumbar level; PCS, preganglionic 

 sacral center; PG.TH, preganglionic thoracic center; P.V.SY, paravertebral sym- 

 pathetic ganglia; S, sacral level; S.C, superior cervical ganglion; TH, thoracic level. 



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