. The development of the chick; an introduction to embryology . composing themantle layer may be twofold, viz., from the young epithelialcells or from the germinal cells. According to some authorsyoung epithelial cells may be transformed into either neuroblastsor neuroglia cells. Thus the form of the youngest neuroblastsin Fig. 139 indicates derivation from epithelial cells, but this 240 THE DEVELOPMENT OF THE CHICK cannot be regarded as pioved. Similarly intermediate stagesbetween epithelial and tnio glia cells are apparently shown inFig. 148. However, there can be but little doubt that the p
. The development of the chick; an introduction to embryology . composing themantle layer may be twofold, viz., from the young epithelialcells or from the germinal cells. According to some authorsyoung epithelial cells may be transformed into either neuroblastsor neuroglia cells. Thus the form of the youngest neuroblastsin Fig. 139 indicates derivation from epithelial cells, but this 240 THE DEVELOPMENT OF THE CHICK cannot be regarded as pioved. Similarly intermediate stagesbetween epithelial and tnio glia cells are apparently shown inFig. 148. However, there can be but little doubt that the prin-cipal source of the neurol)lasts of the mantle layer is tiie germinalcells, that migrate outwai-ds l)etween the bodies of the epithelialcells. The germinal cells continue in active division up to atleast the eleventh day, and their activity seems sufficient toprovide for all the cellular elements of the mantle layer, whereasthe epithelial cells ap])arently do not divide at all. Moreover,mitoses are not infrequent in some cells of the mantle layer Fig. 143. — Transverse section of the cord of anine-day chick to .show neurogHa and ependynialcolls; prepared liy the method of Goljri. (AfterRamon y Cajal.)D., Dorsal. Ep., ?pendyinal cells. Ngl., Neu-roglia cells, v., Ventral. The form of the gray matter in the cord in successivestages is shown in Figs. 144, 145, and 140, representing sectionsof the cord at five, eight, and twelve days. It will l)e seen thatthe gray matter gains very rajiidly in importance between thefifth and tlu^ eighth days. Attention should he (lirect(Ml to a group of neurohlasts situated atthe external margin of the wliite matt(>r just above the ventral is known as Hoffmanns nucleus; it extends the entire length of thecord (I^ig. 1 4(), twelvi days). The white matter of the cord gains in importance at an equalrate (Figs. 144, 145, 14G). Its production is due to ascending THE NERVOUS SYSTEM 241 and descending tracts of fibers derived
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