SPINAL COED 



101 



the main principle underlying the theory may be maintained, the divergent 

 results of different observers in matters of detail cannot yet be brought into line 

 in any general hypothesis. 



MORPHOGENESIS OF THE SPINAL CORD AND BRAIN. 



SPINAL CORD. By the end of the first month the neural canal in the region oi 

 the trunk shows, instead of the primitive rounded cavity, a narrow cleft-like 

 lumen, which widens somewhat at its dorsal end (fig. 137). This change is 



pr. dors. col. 



pr. dors. col. 



ventr. col. ventr. horn 



ventr. com. 



ventr. horn ventr. col. 



FIG. 138. SPINAL CORD OF A HUMAN EMBRYO OP 15'5 MM. (T. H. Bryce.) 



c<:nt>: com., ventral commissure ; ventr. col., ventral column : ventr. horn, primitive ventral horn; 

 l>f. ilnrn. col., primitive dorsal column; m.r. motor, s.r. sensory root. 



due to thickening of the lateral walls, which shows the three zones already 

 described inner ependymal, middle mantle, and outer reticular ; while the roof 

 and floor are represented merely by the ependymal lamella. The mantle zone 

 shows large numbers of neuroblasts whose nerve-fibre paths tend towards the 

 ventro-lateral aspect and leave the cord as the ventral nerve-roots. By the end 

 of the fifth week (fig. 138) the lumen shows a dorsal and ventral narrowing and 

 a mesial expansion, indicating a later demarcation of the lateral walls into dorsal 

 or alar, and ventral or basal plates. The ependymal roof projects beyond the 

 general surface as a convex swelling, and on each side of this is a portion of the 



