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How is the hypoblast related to the endoderm?
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[QUOTE="BillTre, post: 6468917, member: 581757"] Recently, due to new techniques, people have been able to do a lot more lineage tracing which has in some cases revealed that thighs that looked distinctive morphologically, can in fact change to neighboring cell types. It has resulted in a lot of revisions, some of which you may have run into. I kind of like the weeds Jim! The original was ontogeny recapitulates phylogeny. A rough guide, but with a lot of deviations. One of the Evo-Devo revelations is that this holds less truth than previously presumed. It was thought to be a funnel-like relationship. Embryos were considered more similar at early stages and less so later as they divergently developed into their taxon specific morphologies. It is now widely realized that there are a diversity of pregastrula situations that converge on the post gastrula stages, which best unite different body structures. The early differences are attributed to things like: [LIST] [*]big eggs vs. small eggs (large or small yolk for the embryonic cells to navigate around. [*]yolk isolated in a yolk sac vs. distributed though all the cells of the embryo [*]formation of extra-embryonic structures [/LIST] These differences are thought to have arisen under selective differences favoring things like lots of small (little yolk; some fish can lay more than 100,000 eggs)) eggs or fewer eggs with more yolk (greater individual survival, but requiring greater maternal investment. [/QUOTE]
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How is the hypoblast related to the endoderm?
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