. Cunningham's Text-book of anatomy. Anatomy. 502 THE NEKVOUS SYSTEM. Such fibres are constituent elements of the facial, glosso-pharyngeal, and in some animals also the vagus cerebral nerves (Fig. 443), in connexion with the ganglia of which these epibranchial placodes are formed (Froriep and Streeter). The observations of Professor J. P. Hill upon embryos of Echidna seem to suggest that in mammals these gustatory neuroblasts are derived from the entoderm. When first formed, the neural tube is compressed from side to side and presents an elliptical outline in transverse section (Eig. 444). Th
. Cunningham's Text-book of anatomy. Anatomy. 502 THE NEKVOUS SYSTEM. Such fibres are constituent elements of the facial, glosso-pharyngeal, and in some animals also the vagus cerebral nerves (Fig. 443), in connexion with the ganglia of which these epibranchial placodes are formed (Froriep and Streeter). The observations of Professor J. P. Hill upon embryos of Echidna seem to suggest that in mammals these gustatory neuroblasts are derived from the entoderm. When first formed, the neural tube is compressed from side to side and presents an elliptical outline in transverse section (Eig. 444). The two lateral walls are very thick, whilst the narrow dorsal and ventral portions of the wall are thin, and are termed the roof-plate and floor-plate respectively (Fig. 444). The cavity of the tube in transverse section appears as a narrow slit. The wall of the neural tube consists at first of low columnar epithelium arranged in a fairly regular series, but with a certain number of large spherical so-called germinal cells scattered between the columns. But this regular disposition as a single layer Funiculus posterior Posterior nerve root. Commissural fibre " -Anterior nerve root Fig. 444.—Diagram of Transverse Section of Early Neural Tube. of cells does not last long. For even by the second week the rapid proliferation of the cells has led to a marked increase in the thickness of the lateral wall and a scattering of the more numerous nuclei, apparently irregularly, throughout its substance (Fig. 444). The latter consists of a network of protoplasm in which definite outlines of cells cannot be detected. As growth proceeds the innermost part of this nucleated protoplasmic syncytium becomes condensed to form a delicate membrane termed the internal limiting membrane, which lines the lumen of the tube, whilst its outermost part presents a similar relation to an external limiting membrane, which invests the outer surface of the tube. To- ward the end of the first month the la
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