386 



CHORDATE ANATOMY 



brain regions occur in all vertebrates, and from them all the parts of the 

 adult brain are formed. 



Brain Flexures. While the subdivision of the primary vesicles is 

 taking place in ontogenesis, the brain undergoes, in amniote embryos, three 

 successive flexures, the cephalic or primary, the pontine, and nuchal or 

 cervical. The cephalic flexure occurs in the mid-brain region, the other 

 two in the region of the hind-brain. All three flexures are in a vertical 

 plane, but the bend of the pontine is the reverse of the other two. The 

 bending is presumably the result of the elongation of the brain in limited 

 space, since the brain elongates more rapidly than does the head itself. 

 The cephalic flexure is well marked in the embryos of Anamhia, but the 

 pontine and nuchal flexures scarcely appear. They become increasingly 

 evident as we pass from lower to higher amniotes. (Fig. 340) 



DEVELOPMENT OF THE SPINAL CORD 



When the neural plate becomes the neural tube, its wall is a simple 

 columnar epithelium. In consequence of more rapid cell proHferation, 



erve root 



D. 100mm. E. Adult 



Fig. 341. — -Transverse sections through spinal cord of the pig at various ages. Note 

 especially the parts of the adult cord derived from the ependymal, mantle, and marginal 

 layers of the embryonic neural tube. (From Patten's "Embryology of the Pig.") 



limited to a layer of germinal cells which lie near the lumen of the cord, 

 the lateral walls become greatly thickened, while the mid-dorsal and 

 mid- ventral portions remain as thin roof and floor plates. 



This thickened lateral wall becomes divided by a median longitudinal 

 sulcus into a dorsal alar plate and a ventral basal plate. 



Three layers are differentiated in the lateral walls of the cord, an 

 ependymal layer next to the lumen, a thick mantle layer of spindle-shaped 



