Journal of morphology . Origin of the Ven-tral Plate and of the Mesoblast-bands. ClosureOF THE Blastopore. Development of the Stomo-D^UM. (Plates XVII and XVIII.) We turn now to the history of the lower hemisphere, in thecourse of which the most complicated and interesting events ofthe ontogeny take place. These events are so intimately boundtogether that it will be necessary to give first a brief resume. The boundary between the upper and lower hemispheres isformed by the prototroch which, as has been shown, is a prod-uct of the first group of micromeres. The superficial cells ofthe lower hem
Journal of morphology . Origin of the Ven-tral Plate and of the Mesoblast-bands. ClosureOF THE Blastopore. Development of the Stomo-D^UM. (Plates XVII and XVIII.) We turn now to the history of the lower hemisphere, in thecourse of which the most complicated and interesting events ofthe ontogeny take place. These events are so intimately boundtogether that it will be necessary to give first a brief resume. The boundary between the upper and lower hemispheres isformed by the prototroch which, as has been shown, is a prod-uct of the first group of micromeres. The superficial cells ofthe lower hemisphere are therefore wholly derived from thesecond and third sets of micromeres and from the productsof the second somatoblast. After the formation of the second somatoblast { at the endof the spiral period, Figs. 29, 31, 33, 49) there are in all fourteenof these cells, viz. : — The three smaller secondary micromeres and their progeny(«2-i, «^ ^^ ^^ ^^ ^^_ No. 3.] THE CELL-LINEAGE OF NEREIS. 405. The first somatoblast {X) and its progeny {x^, x^). The third group of micromeres {a^, b^, ^, d^). The second somatoblast {M). The arrangement of these 9l. cells with respect to theblastopore is shown in Dia-gram III. The entomeresare considerably more thanhalf enclosed by the othercells (Figs. 33, 34), and theopen space may be calledthe blastopore. It will beseen from the diagram andfrom a comparison of thefigures, that the blastoporeis somewhat asymmetrical,and that its hinder lip(which is formed by thesecond proteloblast) liessomewhat posterior to thelower pole (which lies inthe middle of the cross-furrow). The closure of the blastopore takes place precisely atthe lower pole (Figs. 55-57) by convergence of the cells from allsides, though the shape and position of the blastopore are suchthat the principal growth of the cells during the closure takesplace from in front backwards { as in Rhynchelmis or Clepsine,and not as in Lumbric7is). The margin of the bl
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Keywords: ., bookauthorwistarin, bookcentury1800, bookdecade1890, bookyear1892
