102 



NERVOUS SYSTEM 



reticular zone, which is connected with the dorsal nerve-roots, and is known as 

 the primitive dorsal column. Ventrally the mantle zone is much expanded, and 

 forms the rudiment of the ventral horn of the grey crescent on each side ; while 

 over this the reticular zone is thickened, and projects as the primitive ventral 

 column. The rudiment of the ventral horn is in the fifth week (fig. 138) sharply 

 marked off from the rest of the mantle zone by the character of its rounded and less 

 deeply staining nuclei. The ventral root-fibres can be seen emerging from the 

 ventro-lateral aspect. Between the horn and the central canal the fibre-paths all tend 

 towards the ventral commissure. Between the dorsal and ventral expansion is a 

 recessed area in which the boundary zone is very thin. In succeeding stages this 



r f' f "'^ : 



FIG. 139. SECTION OF THE SPINAL CORD OF A HUMAN EMBRYO OF 30 MM. (T. H. Bryce.) 



is gradually filled up, until the ventral projection reaches the dorsal root, anc 

 the continuous ventro-lateral column is laid down. The mantle zone of this 

 region becomes the dorsal horn of the grey crescent. It is at first separal 

 from its fellow by the now slit-like dorsal portion of the lumen, but it Ia1 

 becomes isolated as the dorsal septum is laid down. 



The anterior or ventral median fissure is at first a shallow recess between the 

 projecting primitive ventral columns. As these grow in dimensions the lips of 

 the fissure close in to form a narrow cleft, which encloses a process of pia mater 

 developed from the mesenchyme surrounding the cord. At the bottom of the 

 fissure the anterior or ventral commissure is laid down by the extension of nerve- 

 fibres across the middle line. 



