THE NEUKAL TUBE. 



35 



rudimentary in man as contrasted with many other animals. At a later period 

 two transverse bands of fibres appear in the dorsal wall of the diencephalon, one 

 in front of and the other immediately behind the root of the epiphyseal recess. 

 The anterior band is the dorsal or habenular commissure, and the posterior is the 

 posterior commissure of the adult brain. 



These structures, collectively, together with a small diverticulum of the 



Spongioblast 



s Roof-plate 

 Spongioblast 



Floor- plate ' 

 FIG. 46. 



epithelial roof, which appears 

 anterior to the dorsal com- 

 missure, and is called the 

 supra-pineal recess, constitute 

 the so-called epithalamus. 



Each lateral wall of the 

 diencephalon is differentiated 

 into a dorsal and a ventral 

 part. The dorsal part forms 

 a large gray mass called the 

 thalamus, and on the posterior 

 end of the thalamus are de- 

 veloped two rounded eleva- 

 tions the medial and the At Diagram of a transverse section of a spinal medulla which has 

 lateral geniculate bodies, _ not differentiated into groups of cells. 



, . , ... B. Diagram of a transverse section of a spinal medulla showing 



Which Constitute the meta- positions of germinal cells. 



thalamus of the adult brain. 



The ventral or basal portion of the lateral wall of the diencephalon, together 



with the adjacent part of the ventral wall, forms the hypothalamus of the fully 



developed brain. 



The Fate of the Spinal Portion of the Primitive Neural Tube. The 



spinal portion of the neural tube, during the first three months of in tra- uterine 



life, develops equally in its whole extent, but after that period a longer cephalic or 



anterior (superior in the erect posture) and a shorter caudal portion are recognisable. 



The cephalic portion undergoes still further development and is converted into 



the spinal medulla of the adult, 

 but in the smaller caudal or 

 posterior portion retrogressive 

 changes occur, and it is trans- 

 formed into the non-functional 

 filum terminale of the completed 

 medulla spinalis. 



Histological Differentiation 

 of the Walls of the Neural Tube. 

 In the earliest stages of its de- 

 velopment the walls of the neural 

 tube consist of a mass of nucleated 

 protoplasm, more or less distinctly 

 differentiated into cell areas, of 

 columnar form, which extend be- 

 tween and are connected with an 

 internal limiting membrane, bound- 

 ing the neural canal, and an ex- 

 ternal limiting membrane, which 

 surrounds the whole tube. At 



Columnar cells of roof-plate 



Peripheral layer - - 



Neuroblasts 



Mantle layer 



Spongioblast 

 Ependyma cells 



Neuro blast ;~ 



Columnar cells of floor-plate 

 FIG. 47. SHOWING ELEMENTS OF CENTRAL NERVOUS SYSTEM. 



this time the outline of a transverse section of the primitive neural tube is 

 somewhat ovoid. The cavity of the tube is compressed laterally into a dorsti- 

 ventral cleft, which is bounded by dorsal, ventral, and lateral walls. In the 

 dorsal and ventral walls, called respectively the roof- and floor-plates, the columnar 

 character of the primitive epithelial elements of the medulla spinalis is retained 

 throughout the whole of life, but the peripheral parts of some of the cells are 

 converted into fibrils. 



In the lateral walls of the embryonic medulla spinalis some of the cells soon 



