SPINAL CORD. 



427 



life the neural tube, in the spinal region, is divided into a ventral 

 and a dorsal zone by a lateral groove on either side, continuous 

 with that formed in the rhombencephalon two weeks earlier. 

 The tube presents externally, opposite to each lateral groove, 

 a furrow in the mantle layer representing the concavity of the 

 gray crescent and called the central fissure; it is occupied later 

 by the lateral pyramidal and other tracts. The whole of the spinal 

 cord ventral to the bases of the posterior columnar of gray matter 

 is represented by the ventral zones. From them grow out the 



<fA 



Fig. 127. Transverse sections through the spinal cords of embryos of (A) about 

 four and a half weeks and (B) about three months. (McMurrich after His.) 



cB. Fasciculus cuneatus (Burdachi). cG. Fasciculus gracilis (Golli). dh. Posterior 

 columna. dz. Dorsal zone. fp. Floor-plate, ob. Oval bundle (Hisi). rp. Roof -plate. 

 vh. Anterior columna. vz. Ventral zone. 



anterior roots of the spinal nerves. In the dorsal zones the pos- 

 terior columnae and the posterior columns of fibers are developed. 

 They receive the posterior roots of the spinal nerves (Figs. 117, 

 127 and 128). 



Three Histologic Layers (Figs. 116 and I27A). At a time 

 somewhat earlier than the division into ventral and dorsal zones, 

 even at the fourth or fifth week, the spinal part of the neural tube 



