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Cleavage of Mesolecithal Eggs
Amphibians
Amphibians typically exhibit mesolecithal eggs, having an intermediate amount of yolk. The amphibian egg is larger than that of the starfish due to the larger quantity of yolk. It is distinctly divided into a dark pigmented hemisphere (the animal hemisphere) and a lightly or unpigmented hemisphere (the vegetal hemisphere). The pigment granules reside in the cortical cytoplasm of the egg within the plasma membrane and are more abundant in the animal hemisphere. The yolk is distributed throughout the cytoplasm of the egg but is much more concentrated toward the vegetal pole end of the egg. Hence, the yolk offers resistance to cleavage in the vegetal hemisphere. The amount of yolk is still sufficiently small to allow complete cleavage of the egg cell.
Scroll down the page for illustrations and photomicrographs of selected stages of early frog embryos.
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Drawing of the 8 Cell Frog Embryo showing Distribution of Pigment and Cleavage Divisions The first cleavage was vertical through the animal and vegetal poles. The second cleavage was also vertical, but at right angles to the first. The third cleavage was horizontal but unequal. The embryo now consists of four smaller blastomeres (micromeres) at the animal pole and four larger blastomeres (macromeres) at the vegetal pole. AP = animal pole, VP = vegetal pole. |
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The 8 Cell Frog Embryo A - Diagram - Surface View, B - Sagittal section Diagram A shows the plane of sectioning of photomicrograph B. In the sagittal section (B), note the smaller blastomeres (micromeres) near the animal pole (AP) and the larger blastomeres (macromeres) near the vegetal pole (VP). Note the presence and location of dark brown/black pigment granules marked by the pink arrows. |
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Frog Blastula - Surface View AP = animal pole, VP = vegetal pole |
Frog Blastula - Sagittal Section AP = animal pole, VP = vegetal pole Bc = blastocoel |
Proceed to Cleavage of Megalecithal Eggs
View Gastrulation in the Mesolethical Embryo
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Department of Integrative Biology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.
