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This page discusses the gastrulation of mesolecithal embryos, using amphibians as an example.
Because of the greater yolk stores and the smaller blastocoel in amphibians, gastrulation is not as straightforward as it is in echinoderms. The principle of moving surface cells into the interior to form new layers is the same, however the method by which this is achieved is different. Just as the large amounts of yolk were an impediment to cleavage, so the large yolky cells impede gastrulation as well.
Gastrulation begins with the invagination (the inpocketing of a layer of cells like the surface of a balloon when poked) of the advancing germ ring cells and continues in a process called involution. Involution is the inturning or inward movement of the outer layer of cells so that it spreads out under the internal surface of the outer layer of cells. Cells at the margin of the germ ring, well below the equator of the blastula, change shape so that the surface layer turns inward and then begins to migrate anteriorly along the roof of the blastocoel. This invagination of migrating cells creates the beginning of the archenteron. The mouth of the inpocketing is the blastopore and the upper margin of the curved fold is known as the dorsal lip of the blastopore. As the in-pocketing deepens the opening extends progressively into a crescent, then into a horseshoe, and finally into a complete circle. As the processes of invagination and involution occur, epiboly continues as the germ ring advances over the exposed yolk cells, so that by the time the circle is complete, the yolk can only be seen within the confines of the blastopore, in an area known as the yolk plug. The blastopore is now surrounded by dorsal, lateral and ventral lips. Involution first occurred at the dorsal lip near the more rapidly dividing animal pole. The expanding layer of cells which forms the roof of the archenteron becomes the endoderm and chordamesoderm. For some time the floor of the archenteron remains composed of large yolk cells. The cells on the outer surface of the embryo now form the ectoderm. As gastrulation continues involution begins to occur at the ventral lip of the blastopore and gradually the invaginated endoderm proliferates to enclose the yolk and complete the archenteron. The third layer, the mesoderm, soon develops between the ectoderm and endoderm. Note that the precise details of the formation of the mesoderm and endoderm varies among amphibians.
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Frog Embryo - Late Blastula Stage AP - Animal Pole On the left is a drawing of an external view. On the right is a diagram of a sectional view. |
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Frog Embryo -Early Gastrula Stage Note that the blastopore (Bp) is crescent shaped and its dorsal lip (upper lip in this image) has begun to form. AP - Animal Pole Left - external view; Right - sectional view |
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Frog Embryo - Mid Gastrula Stage Above Left - drawing of external view. Micromeres of the animal pole continue to overgrow the macromeres of the vegetal pole. The blastopore which first appeared crescent-shaped, becomes horseshoe-shaped, and eventually circular. Above Center - tissue section of mid gastrula showing the formation of a new cavity, the archenteron (A) and the displacement of the blastocoel (Bc). Note that the appearance of the blastocoel is exaggerated due to shrinkage of cells during fixation and the subsequent processing of the specimen for sectioning. Above Right - sectional diagram showing the blastopore (Bp), archenteron (A), and blastocoel (Bc). AP = Animal Pole; VP = Vegetal Pole |
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Frog Embryo - Late Gastrula Above Left - drawing of external view showing the circular-shaped blastopore and "yolk plug" of vegetal pole macromeres. Above Center - sagittal section of a gastula showing the archenteron (A), the dorsal lip of the blastopore (1), the yolk plug (2), and the ventral lip (3) of the blastopore. Note that the blastocoel has been obliterated. Above Right - sectional diagram which has been coloured to show the three germ layers. Blue = ectoderm ; Red = mesoderm; Yellow = endoderm; A = archenteron; Bp = blastopore; YP = yolk plug |
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Review Cleavage in Mesolecithal Eggs
Proceed to Gastrulation in Megalecithal Embryos
Glossary (use browser "Back" button to return)
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Send comments to:
Sandra K. Ackerley (),
Department of Zoology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.