Bio Lab 7 Quiz - Early Stages of Cleavage Microlecithal...

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Early Stages of Cleavage Microlecithal eggs always divide into complete daughter cells, or equal holoblastic. The first cleavage furrow passes through both the animal and vegetal poles and cuts the egg into two equal sized blastomeres (cells). The second cleavage is at right angles to the first. The third pair of cleavages are horizontal and produce eight blastomeres. At this stage it is possible to distinguish between the two poles since the four blastomeres of the animal pole are slightly smaller than those of vegetal half of the embryo. Later Stages of Cleavage During the early period of cleavage, the blastomeres are packed together like a cluster of mulberries, and the embryo is referred to as the morulla. As cleavage continues, a fluid filled blastocoel forms and the blastomeres become displaced into an organized peripheral monolayer of cells; the embryo is now referred to as a blastula. Blastula and Gastrulas You can differentiate early from late blastulas by the size of the blastocoel. Gastrulation, formation of the three germ layers, in echinoderms is not restricted by the present of yolk. The stages of gastrulation can be distinguished by the degree of invagination. The first sign of gastrulation is the flattening of the blastula into a cup shaped gastrula. During gastrolation, the blastocoel is obliterated and a new cavity, the archaneteron is formed. The opening into the archenteron, the blastopore , develops into the anus in deuterostomes. The egg, blastula, and gastrula are approximately equal in size. The individual cells of this embryo are much smaller than the fertilized egg. This is because cleavage, unlike ordinary cytokenisis, occurs so rapidly that the blastomeres lack a growth phase.
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