. Text-book of embryology. Embryology. 92 INVERTEBEATA stom Fig. 73.—Optical section of embryo of Berrie fOTskalii showing the begiiniing of the eiido- dermjil cavities. (After Clmn.) end, parencliynia-like endoderm ;, r, rib ; stom, stomo- daeum whicli forms the so-called *' ; octants likewise throw oPf a smallest micromere, there arises in this way a crown of smallest micromeres surrounding one pole of the embryo. At this stage the embryo consists of fifty-six cells; fnd eight macromeres, and forty- eight micromeres. The eight macromeres now divide each into two cells of
. Text-book of embryology. Embryology. 92 INVERTEBEATA stom Fig. 73.—Optical section of embryo of Berrie fOTskalii showing the begiiniing of the eiido- dermjil cavities. (After Clmn.) end, parencliynia-like endoderm ;, r, rib ; stom, stomo- daeum whicli forms the so-called *' ; octants likewise throw oPf a smallest micromere, there arises in this way a crown of smallest micromeres surrounding one pole of the embryo. At this stage the embryo consists of fifty-six cells; fnd eight macromeres, and forty- eight micromeres. The eight macromeres now divide each into two cells of equal size, so as to form a circle of sixteen large cells, and no further divisions take place in them until the micromeres have completed their multiplication. In subsequent divisions of the micromeres, in the outer octants, the smaller cells divide first and then the larger micromeres divide. In the middle octants also the four smallest micromeres divide first, whilst the eight larger micromeres flatten out and commence to spread over the macromeres, after which they also divide. From this time on, division of the micromeres occurs rapidly, and the sheet of cells to which they give rise spreads more and more over the surface of the macromeres. The macromeres are originally exposed both above and below but the upper opening is nar- rowed as the edge of the ring of smallest micromeres extends inwards, whilst the micromeres rapidly downwards over the surface of the macromeres. This over- spreading of the macromeres by smaller cells is termed epibole. It is a process often met with in the development of animals and is to be regarded as a variety of gastrulation. There can now be seen, radiating from the upper pole, four streaks of specially small and rapidly dividing cells; these are the forerunners of the " ribs" of the adult which carry the comb-like plates of ciha. Each streak corresponds to a pair of ribs. larger extend. Please note that these images are extracted
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
