A system of obstetrics . l: DFP, mesoblastic layer of Intestinal wall; re, commen-cing pleuroperitoneal cavity; n, notochord. the fourth, third, and lateral ventricles of the brain and the passagesuniting them. Formation of the Head-fold.—Soon after the medullary folds haveappeared that portion of the blastoderm which lies near their anteriorend begins to rise up in the form of a fold above the general level oftie resl f the blastoderm. It carries, of course, the medullary groovewith it. (See Fig. 29.) After the front end of the groove is closedthe brain part of the neural canal lies above thi
A system of obstetrics . l: DFP, mesoblastic layer of Intestinal wall; re, commen-cing pleuroperitoneal cavity; n, notochord. the fourth, third, and lateral ventricles of the brain and the passagesuniting them. Formation of the Head-fold.—Soon after the medullary folds haveappeared that portion of the blastoderm which lies near their anteriorend begins to rise up in the form of a fold above the general level oftie resl f the blastoderm. It carries, of course, the medullary groovewith it. (See Fig. 29.) After the front end of the groove is closedthe brain part of the neural canal lies above this tucked-up or folded-off piece of the blastoderm, which is named the head-fold. The THE EARLY STAGES OF DEVELOPMENT 113 diagram (Fig. 34; will aid in understanding the relations of the head-fold at this time. The figure represents a longitudinal section of theembryo soon after the appearance of the head-fold, but before the hinderend of the neural canal is closed in: nc is the neural canal with lining Fig. A Longitudinal Section through an Embryo, after the head-fold has appeared and the medul-fc lary folds have met and closed in the front end of the neural canal, but are still divergentbehind; a cross-section at y would give Fig. 33 (diagrammatic). If the medullary folds inFig. 29 be imagined to have completely met and fused in the cranial, cervical, and anteriordorsal regions, Fig. 34 maybe taken as the diagrammatic longitudinal section of the embryorepresented in Fig. 2J. H, head-fold; ep, external epiblast; ne, neural canal, lined by invo-luted epiblast; x, the point at which the medullary folds diverge and the neural canal is stillopen; m, mesoblast behind the embryonic region of the blastoderm; n, notochord; mmesoblast in front of embryonic region of the blastoderm; hy, hypoblast; al, beginning ofalimentary canal (pharynx). epiblast of its roof continuous still at its posterior opening with theexternal epiblast, and the lining epiblast of its floor continuous with
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
