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The process of gastrulation is affected, as was cleavage, by the amount and the distribution of yolk. As a result, gastrulation differs between different species in details, but the principle is the same in all these species. During gastrulation, some of the cells from the surface of the embryo move to the interior and replicate, thereby forming new layers of cells. These movements are coupled with the differentiation of the migrating cells (caused by the differential activity of certain genes) into histologically unique layers.
During microlecithal gastrulation, a blastula with a relatively large blastocoel is usually formed. As a result, there is plenty of space to move some of the surface cells to the inside to form new layers. This is first seen as the starfish embryo becomes flattened at the vegetal pole (the initial migration and differentiation of cells), which will then invaginate within the blastocoel. When this occurs, the internal layer of cells formed (called the hypoblast or mesendoderm) are the cells which gives rise to the endoderm and mesoderm germ layers. The cells who remain on the outer surface of the gastrula become the epiblast (ectoderm). The cavity created by the invagination is called the gastrocoel or archenteron (primitive gut), whose opening at the vegetal pole is called the blastopore. The anterior end of the hypoblast (mesendoderm) develops out-pocketings (evaginations) on each side to form the mesoderm (mesodermal pouches). These pouches are also called enterocoelic pouches and the cavities within them are called the coelomic cavities. As the embryo grows during gastrulation, cells also bud off the hypoblast and migrate into the blastocoel to form mesenchymal cells.
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| This is the early starfish gastrula. The vegetal pole (VP) is showing the first signs of invagination. AP=animal pole, VP=vegetal pole. | This shows the early gastrula. The beginning of the invagination which will form the gastrocoel (a.k.a. archenteron) is obvious now, as it invaginates anteriorly (toward the animal pole (AP)) from the vegetal pole (VP), where the blastopore is formed. |
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| Here we see the mid gastrula. The gastrocoel continues to develop as the embryo grows. AP=animal pole, VP=vegetal pole. | This is the late gastrula. This picture is a dorsal view of the embryo, with anterior being toward the animal pole (AP) and posterior toward the vegetal pole (VP). Note the beginning of the formation of enterocoelic pouches on the right and left side of the anterior end of the gastrocoel. | |||||
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C O L O U R I Z E !!  |
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| This is the early bipinnaria larva seen in lateral view (from the side). The dorsal aspect of the embryo is facing right and the ventral aspect is on the left side of this picture. The blastopore is at the posterior end (bottom) and is now developing into the anus, while the anterior end of the gastrocoel has bent toward the ventral aspect of the embryo, where a mouth has formed. | Here we see the same picture as
on the right, but now labelled and in glorious technicolour!
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| Top of the page |
| Proceed to Gastrulation in Mesolethical Embryos |
| Return to Gastrulation Index |
| Review Cleavage of Microlethical Eggs |
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Special thanks to Hans Christoffersen for the preparation of this page and to Genevieve Babcock for updating this page. Latest update March 7, 2000.
Send comments to:
Sandra K. Ackerley (),
Department of Zoology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.