The development of the human body; a manual of human embryology . o Fig. dlagrams showing theManner of the Closure of theMedullary Groove. n6 THE DEVELOPMENT OF THE HUMAN BODY. After the separation of the plates of gastral mesodermthe chorda endoderm, which is at first a flat band,becomes somewhat curved (Fig. 57, A), so that it isconcave on its under surface, and, the curvature increas-ing, the edges of the plate come into contact and finallyfuse together (Fig. 57, B), the edges of the primary endo-derm at the same time uniting beneath the chordal tubeso formed, so this layer becomes a c
The development of the human body; a manual of human embryology . o Fig. dlagrams showing theManner of the Closure of theMedullary Groove. n6 THE DEVELOPMENT OF THE HUMAN BODY. After the separation of the plates of gastral mesodermthe chorda endoderm, which is at first a flat band,becomes somewhat curved (Fig. 57, A), so that it isconcave on its under surface, and, the curvature increas-ing, the edges of the plate come into contact and finallyfuse together (Fig. 57, B), the edges of the primary endo-derm at the same time uniting beneath the chordal tubeso formed, so this layer becomes a continuous sheet, as itwas at its first appearance. The lumen which is at first. Fig. 57.—Transverse Sections through Mole Embryos, showing the Formation of the Notochord. ec, Ectoderm; en, endoderm; m, mesoderm; nc, notochord.—(Heapc.) present in the chordal tube is soon obliterated by the en-largement of the cells which bound it, and these cells laterundergo a peculiar transformation whereby the chordaltube is converted into a solid elastic rod surrounded by acuticular sheath secreted by the cells. The notochord liesat first immediately beneath the median line of the med-ullary groove, between the ectoderm and the endoderm,and has on either side of it the mesodermal plates. It is atemporary structure of which only rudiments persist in THE MESODERMIC SOMITES. I I 7 the adult condition in man, but it is a structure character-istic of all vertebrate embryos and persists to a more orless perfect extent in many of the fishes, being indeed theonly axial skeleton possessed by Amphioxus. In thehigher vertebrates it is almost completely replaced by thevertebral colum
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
