Stages in the Early Development of the Frog Embryo

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From where are the micromeres dervived? Why are they much smaller than the macromeres? Micromeres are derived from 3rd cleavage which produces 4 internal and external E blastomeres. It will then undergo unequal cleavage to form micromere. The first two cleavage planes are vertical and divide the egg into four equal blastomeres, designated as macromeres, the third cleavage plane is horizontal but is shifted toward the animal pole so that the upper set of four blastomeres or micromeres is considerably smaller than those below. How many stages of cleavage can you identify? 2-cell stage (3/4 h): The first cleavage furrow, ending the first zygotic cell cycle, is vertically oriented, as is usual until the 32-cell stage. The furrow arises near the animal pole and progresses rapidly towards the vegetal pole, passing through only the blastodisc and not the yolky region of the egg. Near the bottom of the blastodisc the furrow changes to a horizontal orientation to undercut the blastodisc in the fashion described originally by Wilson (1889) for the sea bass, but still leaves the cells only partly cleaved from the underlying yolky region. The two blastomeres are of equal size and appear otherwise undistinguished from one another. 4-cell stage (1 h): The two blastomeres cleave incompletely and in a single plane that passes through the animal pole at right angles to the plane of the first cleavage. Hence, cycle 3 begins with 4 blastomeres in a 2x2 array. A view from the animal pole reveals that the blastodisc is ellipsoidal in shape. The second cleavage plane is oriented along the longer axis. 8-cell stage (1 1/4 h): Cleavages ending cycle 3, still incomplete, occur in two separate planes, parallel to the first one, and on either side of it. They cut the blastodisc into a 2x4 array of blastomeres. As the dechorionated embryo usually lies in a dish, the 4-cell
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