THE NERVOUS SYSTEM. 



479 



lively small ground bundles of the dorsal funiculus. During this period 

 of development of the dorsal portions of the lateral walls the latter have ap- 

 proached each other, reducing the dorsal part of the lumen to a slit. The 

 roof plate has undergone a slight infolding (Fig. 406). Ventral to the dorsal 

 roots there is a groove running along each side of the cord (marginal furrow of 

 His). At four and one-half weeks the number of fibers of the ventro-lateral 

 funiculus has greatly increased and another groove has appeared parallel and 

 ventral to the marginal furrow and forming the dorsal boundary of the ventro- 



Intermediate plate 

 Central canal 



Floor plate - -^ 



Vent. long, sulcus 



Dors, funiculus 



Dors, gray column (post, horn) 



Dors, root 

 Marginal furrow 

 Cylinder furrow 



- Lat. gray column (lat. horn) 

 ^i^/-/- Ventro-lat. funiculus 



Vent, gray column (ant. horn) 

 ^ Vent, root 



Vent, funiculus 

 (ant. white column) 



Vent. sp. artery 



FIG. 406. Half of a transverse section of the spinal cord of a human embryo 

 of 18.5 mm. (7^ weeks). His. 



lateral funiculus (cylinder furrow of His) (Figs. 405 and 406). The portion 

 of the lateral wall lying between these two grooves or furrows forms an 

 intermediate plate which contains few fibers in its marginal layer at this 

 period, and is thus backward in development. Grooves appear on the luminal 

 wall, apparently corresponding approximately to the outer grooves. 



The further growth of the dorsal funiculi and the concomitant growth 

 of the associated gray matter, i.e., of the cells of the adjoining mantle layer, 

 proceed until we have the conditions shown in Figs. 406 and 407. At the 

 same time there is a further approximation of the dorsal portions of the lateral 



