Quain's elements of anatomy . iddle line, but the fibres from theroots of the nerves when formed are traceable into the grey substance oftheir respective anterior and posterior columns. The white substance is formed external to or on the surface of thedeeper grey substance; but it is not yet determined whether it is Avhollydeveloped out of the cehs composing the grey matter or from separateblastema to which the mesoblast may in part contribute. It is certainlycombined with connective tissue elements, and its mode of formation isdifierent from that of the grey substance, which is the more direc
Quain's elements of anatomy . iddle line, but the fibres from theroots of the nerves when formed are traceable into the grey substance oftheir respective anterior and posterior columns. The white substance is formed external to or on the surface of thedeeper grey substance; but it is not yet determined whether it is Avhollydeveloped out of the cehs composing the grey matter or from separateblastema to which the mesoblast may in part contribute. It is certainlycombined with connective tissue elements, and its mode of formation isdifierent from that of the grey substance, which is the more direct productof differentiation of the involuted epiblastic cells. How far the mesoblasttakes part in the formation of the latter is .still doubtful (see p. 192). On the fifth and sixth days in the chick, according to Foster andBalfour, the white columns increase rapidly in size, and the anteriormedian fissure begins to be formed between the anterior columns bytheir sweUing outwards and leaving its interval between them. It is at. 822 THE NERVOUS SYSTEM, first wide and shallow, and soon receiyes a lining of vascular connectivetissue or pia mater. The commissures are now also formed ; the anteriorgrey commissure first, then the posterior grey, and somewhat later theanterior white commissure. Fiff. 724.
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