. Cunningham's Text-book of anatomy. Anatomy. THE NEUKAL TUBE. 35 Spongioblast ⢠Roof-plate Spongioblast- rudimentary m 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 epithelial roof, which appears anterior to the dorsal com-
. Cunningham's Text-book of anatomy. Anatomy. THE NEUKAL TUBE. 35 Spongioblast ⢠Roof-plate Spongioblast- rudimentary m 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 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. Floor-plate A Floor-plate ' B Fig. 46. and the &~ Diagram of a transverse section of a spinal medulla which lias lateral geniculate bodies, . not differentiated into groups of cells. â U- i, â,-. +-4-âiâ j-i ^ j. 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.âThe spinal portion of the neural tube, during the first three months of intra-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
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