196 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



(1) Some become neuroglia, elongated and branched supporting 

 cells ; in the outer zone of the tube wall they form a mesh- 

 work ; in the inner zone, round the central canal, they form the 

 ependyma; in the middle zone their interstices are filled with 

 neuroblasts. 



(2) Some of the cells become differentiated into neuroblasts, 

 These are produced from germinal cells lying in or near the inner 

 zone (Fig. 160). Two points should be noted (Fig. 160) — 



(a) That the cells in the roof and floor scarcely undergo any 

 proliferation — they retain for a considerable time the 

 simple embryonic form. 



(I) That there are two lateral centres of proliferation — a dorsal 

 centre or lamina, the cells of which are connected after- 

 wards with the ingrowing fibres of the dorsal root, and 

 a ventral centre or lamina, at which the cells of the 

 anterior horn are produced and from which the fibres 

 of the anterior root arise. 



outer zone 

 middle zone 



inner zone 



post mes. (from post, roots) 



post. lat. (from post, roots) 

 ^.margin, (from post, roots) 



crossed pyram. (from 

 "motor cortex) 

 _a.sc. cerebellar 

 (from ant. horn) 



qsc. cerebellar 

 (from Clark's col.) 



desc. cerebellar (from 

 cerebellum) 



ant.pyram (from motor cortex) 



Pig. 161.— Diagrammatic Section o£ the Spinal Cord to show the Parts formed in the 

 three Zones of the Embryonic Spinal Cord. 



The changes which occur in the Spinal Cord after the 6th week 

 (see Figs. 160 and 161) are: 

 A. In the Outer Neuroglial Zone : 

 (1) Postero-mesial and postero-lateral columns are produced 



