. The science and art of midwifery. the dorsalplates are seen rising up as low ridges. At the same time, just be-neath the furrow, a cylindrical organ, known as the chorda dorsalis,becomes separated from the cell-mass. The chorda dorsalis owes itsimportance to the fact that it is around this cylindrical body that thevertebrae subsequently form. The vertebrae themselves are derivedfrom two longitudinal chords,separated by a cleavage from theportions of the intermediate layernext to either side of the chordadorsalis. The peripheral portionsof the intermediate layer are nowtermed the lateral or a
. The science and art of midwifery. the dorsalplates are seen rising up as low ridges. At the same time, just be-neath the furrow, a cylindrical organ, known as the chorda dorsalis,becomes separated from the cell-mass. The chorda dorsalis owes itsimportance to the fact that it is around this cylindrical body that thevertebrae subsequently form. The vertebrae themselves are derivedfrom two longitudinal chords,separated by a cleavage from theportions of the intermediate layernext to either side of the chordadorsalis. The peripheral portionsof the intermediate layer are nowtermed the lateral or abdominalplates. Meantime the dorsalplates continue to grow, and, bycurving toward one another, final-ly meet in the median line, so as to form a closed tube, the tubusmedullaris, in which is developed the central nervous system. Thusit will be noticed that the organ through the agency of which theindividual is brought into contact with the external world is primi-tively derived from the external blastodermic layer (ectoderm). t Tib. Fig. 41. — Diagram representing transversesection through the embryo of a chick atthe end of the first day of incubation,ra, dorsal plates ; eh, chorda dorsalis ; r,vertebral chords ; a p, abdominal plates.
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Keywords: ., bookcentury1800, bookdecade1890, bookidsci, booksubjectobstetrics
