THE NERVOUS SYSTEM. 



453 



plates or zones : A ventral median plate (floor plate] , a dorsal median plate (roof 

 plate], where the fusion occurred, and two lateral plates (e.g., Fig. 442). 



Two points are to be noted: First, that the neural plate is a bilateral struc- 

 ture and the future development of the tube will naturally take place principally 

 in the side walls or lateral plates of the formed tube; second, that the primary 

 connection between the two side walls is the ventral median plate, the dorsal 

 median plate having been produced by a secondary fusion. This being the 

 case, the ventral connection between the two lateral plates will naturally be 

 more extensive and possibly more primitive than the dorsal. The ventral and 

 dorsal median plates do not usually develop nervous tissue, but bands of vertical 

 elongated ependyma cells. In places the roof plate expands into thin mem- 

 branes which are covered with vascular mesodermal tissue forming chorioid 

 plexuses, such as the chorioid plexuses of the lateral, third and fourth ventricles 

 (Fig. 408). 



FIG. 400. Scheme of a median sagittal section through a vertebrate brain before 



the closure of the neuropore. von Kupffer, 



A., Archencephalon; >., deuterencephalon; Ms., medulla spinalis (spinal cord); cd., notochord; 

 en., neuronteric canal; ek., ectoderm; en., entodernv /., infundibulum; up., neuropore; pv. t 

 ventral cephalic fold; tp., tuberculum posterius. 



It has already been seen that even at its first appearance the neural plate 

 exhibits a differentiation into an anterior expanded part, the brain, and a 

 posterior narrower part, the spinal cord. After closure, in many Vertebrates at 

 least, a three-fold division can be made out: (i) A caudal part of the neural 

 tube, the spinal cord, which gradually expands cranially into (2) the caudal part 

 of the brain (deuterencephalon, v. Kupffer) (Fig. 400). These two parts lie 

 above the notochord and all the typical cerebrospinal nerves are connected 

 with them. (3) Cranially, at the anterior end of the notochord, the brain wall 

 expands ventrally forming the third portion (archencephalon) . At the forward 

 extremity is seen the anterior neuropore. The deuterencephalon is thus an 

 epichordal part of the brain, while the archencephalon is prechordal. At the 

 boundary between the two is a ventral infolding of the brain wall the ventral 

 cephalic fold (plica encephali ventralis) . At this stage the brain resembles that 

 of Amphioxus in many respects. From each side w r all of the archencephalon 



