Text-book of comparative anatomy . After the micro-meres have thus grown round the macromeres, leaving a large region at the vegeta-tive pole in which the macromeres come freely to the surface, the formation ofmicromeres does not cease, as in the Polyclada. The already formed micromeres,however, yield exclusively ectoderm ; the remaining macromeres, part of the mesodermand the endoderm. Here also no very recognisable gastrula is developed. A. FIG. 95.—Three stages in the segmentation of a Ctenophoran egg. mi, Micromeres; ma, macromeres. In all processes of the formation of micromeres the follo
Text-book of comparative anatomy . After the micro-meres have thus grown round the macromeres, leaving a large region at the vegeta-tive pole in which the macromeres come freely to the surface, the formation ofmicromeres does not cease, as in the Polyclada. The already formed micromeres,however, yield exclusively ectoderm ; the remaining macromeres, part of the mesodermand the endoderm. Here also no very recognisable gastrula is developed. A. FIG. 95.—Three stages in the segmentation of a Ctenophoran egg. mi, Micromeres; ma, macromeres. In all processes of the formation of micromeres the following is to be speciallynoted. After a macromere has constricted off a micromere, or, what is the samething, after a blastomere has divided into a small micromere with little or no nutri-tive yolk, and into a large macromere with much nutritive yolk, the portion offormative yolk or protoplasm which remains in the macromere grows evidently bythe assimilation of nutritive yolk before the macromere can again divide. 3. Segmentation and gastrulation of the holoblastic centrolecithal germ. as an example of this the Gcryonid germ which has been the most carefullyinvestigated and is the best understood (Fig. 96). We are already acquainted withthe 8-blastomere stage. Each blastomere is telolecithal, with deutoplasm directedtowards the centre of the germ and protoplasm towards the circumference. The8 blastomeres divide into 16, and
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
