. Quain's elements of anatomy . rimitive groove, closed in front bythe coalescence of the two lateral ridges ;mc, 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 ; \x\ front the fold of the amnion iscommencing. 2. Medullary or Neural Cen-tre.—The first rudiment of the greatneural centre makes its appearancesoraeAvhat later than the primitivestreak, viz., in the chick about theeighteenth hour, and in the rabbitsovum on the seventh day. It takesplace entirely within the
. Quain's elements of anatomy . rimitive groove, closed in front bythe coalescence of the two lateral ridges ;mc, 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 ; \x\ front the fold of the amnion iscommencing. 2. Medullary or Neural Cen-tre.—The first rudiment of the greatneural centre makes its appearancesoraeAvhat 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 samestreak so as to occupy that part of the area NEUEAL CENTER 789 The medullary plates which result from the thickening of the epihlastare at first comparatively short, coiTesponding mainly to that which willultimately become the cephalic part of the nervous centre. They soon. Fig. Fig. 681.—Embryonic area, with outline of the vascular area, frou a rabbits0VU3I OF SEVEN DAYS. \8 (From Kolliker.)00, vascular area; ag, embryonic area ; jpr, primitive streak and groove; rf, medullary however, elongate, and increasing in their thickness at the outer margins,they rise into lateral ridges, which are separated by a groove or furroAV,whUe they run into one another, or are joined together round thecephalic extremity of the groove. Behind they diverge somewhat, and Pig. 682.—Dorsal view of a blastoderm and Fig. 682. embryo chick HAviNa five mesoblastic somites.• (From Balfour.) , anterior part of the primitive streak ; ,posterior part; the medullary ridges have come to-gether in the greater part of their extent, but havenot yet united ; the caudal swellings are visible on•each side of there they extend for some distance on eitherside of the primitive streak, so as to enclosenearly its anterior half between them. As the
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