THE NERVOUS SYSTEM 



517 



walls so that the widest part of the lumen is further ventral. At about eight 

 weeks the portion of the wall near the median line, which has formed a ridge 

 by the apposition of the two inner layers and the roof plate (Fig. 444 Y), and is 

 uncovered as yet with fibers, differentiates a marginal layer (eight and one-half 

 weeks, Fig. 445) into which fibers grow forming, on each side, in the upper 

 part of the cord, the column of Goll or fasciculus gracilis (Fig. 446) . Many 

 of these fibers, at least, are the ascending arms of caudal dorsal root fibers, 

 which are thus added mesially to the continuations of upper cord roots. It will 



Rudiment of funiculus gracilis 



Dorsal funiculus (cuneatus) 



Intermediate plate 



Central canal 



Dors, gray column 



Dors, root 

 Marginal furrow 

 Cylinder furrow 



Lat. gray column 

 Ventro-lat. funiculus 



. , - Vent, gray column 



Floor plate - - >%Pgzsg2i> 



Vent. long, sulcus f-\ \{f'/ 1 f i l\\ \\\\^J^Sk- Vent, root 



I *« — - — - Vent, funiculus 



Vent. spT art. 



Fig. 445.' — Half of a transverse section of the spinal cord of a human embryo of 

 24 mm. (8J weeks). Sis. 



be noted that there is now a massive dorsal gray column and that the original 

 oval bundle has extended around on the mesial, side of this gray column. 



While these changes are taking place, the dorsal portions of the lateral walls 

 have fused, probably beginning at the most dorsal part, thus forming the dorsal 

 septum. This may be accompanied by a certain amount of rolling in from the 

 dorsal part indicated by the direction of the ependyma cells (Fig. 445) • The 

 growth of the ventral funiculi and gray columns results in the appearance 

 and subsequent increasing depth of the ventral longitudinal fissure. The cord 

 now resembles the' adult cord in many features, having well-marked white* and 



*The term ''white " column is used for convenience. The funiculi do not become "white" until 

 their fibers become myelinated during the sixth month. 



