The hypoblast forms endodermal organs, (stomach, intestine, lungs, liver etc.) The mesoblast cells crawl laterally to either side: most of them will form mesodermal organsīut some mesoblast cells insert themselves into the hypoblast & become part of it Those epiblast cells that undergo ingression, after they have ingressed, are called " mesoblast cells" Next, an elongate thickening of epiblast forms called the " primitive streak" : (this will be body axis).Īlong midline of primitive streak, ingression occurs along line, " primitive groove" instead of blastopore. The epiblast is the upper=outer layer the hypoblast is a thinner layer under it (next to yolk in birds). In bird, reptile & mammal embryos, before gastrulation two layers of epithelial cells have been formed. When gastrulation is by invagination or involution, there is a blastopore where the cells move inside. Note that ingression entails the conversion of the cells from being epithelial to being mesenchymal! Or by internalization/ separation of cells from an epithelium (" ingression "),Īs in mammal, bird and reptile gastrulation, and also as in the formation of the primary mesenchyme in echinoderm gastrulas. Or by rolling as a sheet (" involution") as in amphibian (salamander, newt, frog & toad) gastrulation, Gastrulation is a complex series of active cell rearrangements, especially movement of surface cells into the interior,Įither by folding as a sheet (" invagination" as in sea urchin gastrulation), We have a blastocyst which differs from a blastula in that only some cells in the internal part will become the adult body. ( Placental) mammal (including human) embryos are special at this stage: Biology 104: Spring 2004 : notes from Wednesday, JanuGastrulation:Ĭleavage results in formation of the blastula (hundreds to a few thousand cells=" blastomeres").Īmphibian & urchin blastulas are hollow balls Bird & reptiles are flat sheets of cells at this stage.
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