550 



COMPARATIVE ANATOMY 



the lateral walls become greatly thickened, while the mid-dorsal and 

 midventral portions remain as thin roof and floor plates. 



This thickened lateral wall becomes divided by a median longitudinal 

 sulcus into a dorsal alar plate and a ventral basal plate. 



syncytium 



r'^-m^^ 



- germinal 

 cell 



B 





) germinal 

 ' cells 



■^.iis^ii^ 



n 



mesenchyme 





germinal 

 •/ cells 



DO 



ext. limiting 



membrane 



nuclei of 

 mantle layer 



int. limiting 

 membrane 



marginal 

 layer 



mantle layer 



ependymal 

 layer 



Fig. 453. — Stages in the histogenesis of the spinal cord. A, From open neural plate 

 of rabbit embryo. B, From wall of recently closed neural tube, 5 mm. pig embryo. 

 C, From neural tube of 7 mm. pig. D, From neural tube of 10 mm. pig. (All drawings 

 X550.) (From Patten's "Embryology of the Pig," after Hardesty.) 



Three layers are differentiated in the lateral walls of the cord, an 

 epend)nnal layer next to the lumen, a thick mantle layer of spindle-shaped 

 cells, and an outer marginal layer of fibers free from cells. The marginal 

 layer increases in thickness by the addition of fibers which grow lengthwise 

 of the cord. By the addition of myelin sheaths to these fibers, the 

 marginal zone is converted into the white matter of the cord. 



As the lateral walls increase in thickness, the lumen of the cord is 

 narrowed down so that in a cross section it appears as a slit which, how- 



