The elements of embryology. / By M. Foster ... and Francis M. Balfour.

  • Michael Foster
Date:
1874
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Licence: Public Domain Mark

Credit: The elements of embryology. / By M. Foster ... and Francis M. Balfour. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    observed between it and the actual bead. In other words, a distinct neck has been formed, in which most important changes take place. 1'he neck is distinguished from the trunk in which the heart now lies by the important feature that in it there is no cleavage of the mesoblast into somatopleure and splanch- mopleure, and consequently no pleuroperitoneal cavity. In ipassing from the exterior into the alimentary canal, the three ;]ayers of the blastoderm are successively traversed without lany breach of continuity save such as is caused by the cavities ^of the blood-vessels. In this neck, so constituted, there ap- ^pea^ on the third day certain fissures or clefts, the visceral or abranchial clefts. These are real clefts or slits passing right through the walls of the throat, and are placed in series on ;either side across the axis of the alimentary canal, lying not iquite at right angles to that axis and parallel to each other, but converging somewhat to the middle of the throat in front :(Fig. 37). Viewed from the outside in either fresh or pre- served embryos they are not very distinctly seen to be clefts; >but when they are seen from within after laying open the ithroat, their characters as elongated oval slits can easily be •(recognised. Four in number on either side, the most anterior is the ifirst to be formed, the other three following in succession. ITheir formation takes place from within outwards. The 1 hypoblast and mesoblast are first absorbed along the line 'of the future cleft, then the epiblast is broken through, and the hypoblast, which is carried outwards as a lining to the ;sht, forms a junction with the epiblast at the outside of the ithroat. No sooner has a cleft been formed than its upper border {i e. the border nearer the head) becomes raised into a thick lip or fold, the visceral or branchial fold. Each cleft has its >own fold on its upper border, and in addition the lower •border of the fourth or last visceral cleft is raised into a i similar fold. There are thus five visceral folds to four visceral clefts (Fig. 37). The last two folds however, and ^especially the last, are not nearly so thick and prominent kas the other three, the second being the- broadest and most •(Conspicuous of all. The first fold meets, or nearly meets its ^fellow in the middle line in front, but the second falls short