Quain's elements of anatomy . e cerebro-spinal nervous axis of the embryo. A -4— Fig. 680. Fig. 680.—Surface view of the trans- parent AREA OP A blastoderm OP 18HOURS, SOMEWHAT DIAGRAMMATIC. (From Balfour.) pr, primitive groove, closed in front bythe coalescence of the two lateral ridges ;inc, medullary groove, having on each sidethe medullary folds or ridges, A, which alsomeet in front to enclose the groove, but di-verge behind so as to enclose the primitivestreak ; in front the fold of the amnion iscommencing. 2. Medullary or Neural Cen-tre.—The first rudiment of the greatneural centre makes
Quain's elements of anatomy . e cerebro-spinal nervous axis of the embryo. A -4— Fig. 680. Fig. 680.—Surface view of the trans- parent AREA OP A blastoderm OP 18HOURS, SOMEWHAT DIAGRAMMATIC. (From Balfour.) pr, primitive groove, closed in front bythe coalescence of the two lateral ridges ;inc, medullary groove, having on each sidethe medullary folds or ridges, A, which alsomeet in front to enclose the groove, but di-verge behind so as to enclose the primitivestreak ; in front the fold of the amnion iscommencing. 2. Medullary or Neural Cen-tre.—The first rudiment of the greatneural centre makes its appearancesomewhat later than the primitivestreak, viz., in the chick about theeighteenth hour, and in the rabbitsovum on the seventh day. It takesplace entirely within the epiblast orouter layer, and depends on thegrowth of the cells of that layer inthe anterior part of the embryonicarea where it extends in the same direction as the primitive streak so as to occupy that part of the ai-ea left vacant by the NEUEAL CENTEE. 789 The medullary plates which result from the thickening of the epiblastare at first comparatively short, corresponding mainly to that which willultimately become the cephahc part of the nervous centre. They soon, Fig. 681.
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