Text-book of comparative anatomy . FIG. 93.—Segmentation and gastrulationof the egg of Bonellia, after Spengel. mi,Micromeres ; ma, macromeres ; ec, ectoderm. II SEGMENTATION AND GASTRULATION 125. yield a great part of the mesoderm (musculature, sexual organs, connective tissue),and are overgrown by the ectoderm micromeres. All micromeres which are detachedlater belong to the endoderm. We have here before us a typical case of the tendencyto shift back formations to very early stages of development. A typical gastrula isnot developed in the Polyclada, since theseparation of ectoderm and endoder
Text-book of comparative anatomy . FIG. 93.—Segmentation and gastrulationof the egg of Bonellia, after Spengel. mi,Micromeres ; ma, macromeres ; ec, ectoderm. II SEGMENTATION AND GASTRULATION 125. yield a great part of the mesoderm (musculature, sexual organs, connective tissue),and are overgrown by the ectoderm micromeres. All micromeres which are detachedlater belong to the endoderm. We have here before us a typical case of the tendencyto shift back formations to very early stages of development. A typical gastrula isnot developed in the Polyclada, since theseparation of ectoderm and endoderm occursas early as the 8-blastomere stage. Weshall return to this later. The Ctenophora exhibit an interestingprocess of segmentation and gastrulationintermediate between the gastrulation bymeans of epibole of the telolecithal holo-blastic eggs and the gastrulation by de-lamination of the centrolecithal eggs to be FlG- 94-—Segmentation of a Polyclad spoken of later. The 8-blastomere stage of egg (of DiscoceUf »> Fii;st generation of micromeres (ectoderm-forming cells); mi^, these animals has been described above. seeond genei-ation of micromeres (mesodermAll the 8 blastomeres are te
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
