Text-book of comparative anatomy . the vegetative pole. This processis called epibole ; it is the growth ofmicromeres over a resting mass of verylarge macromeres. It will be seen fromthe above description that this process andthat of invagination are fundamentallyidentical. The gastrula which is formedis a solid sterrogastrula, whose entericcavity is almost filled up by the large sizeof the yolk-laden macromeres. The micro-meres present at this stage form the ectoderm ; the macromeres represent the rudi-ments of the endoderm and of a part of the mesoderm. The blastula stage of thismethod of de
Text-book of comparative anatomy . the vegetative pole. This processis called epibole ; it is the growth ofmicromeres over a resting mass of verylarge macromeres. It will be seen fromthe above description that this process andthat of invagination are fundamentallyidentical. The gastrula which is formedis a solid sterrogastrula, whose entericcavity is almost filled up by the large sizeof the yolk-laden macromeres. The micro-meres present at this stage form the ectoderm ; the macromeres represent the rudi-ments of the endoderm and of a part of the mesoderm. The blastula stage of thismethod of development is unrecognisable. From the type of segmentation exemplified by Bonellia we pass on to one nearlyrelated which occurs in the Polydada (Fig. 94). The formation of micromeres, andtheir growth over the macromeres, is just as in Bonellia. But here the 4 micromereswhich are first constricted yield, by division, the whole ectoderm. The micromeres,which become constricted off from the macromeres in the second (and third) order,. FIG. 93.—Segmentation and gastrulationof the egg of Bonellia, after Spengel. mi,Micromeres ; ma, macromeres ; ec, ectoderm. II SEGMENTATION AND GASTRULATION 125
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
