![]() ![]() All of the activities up to now have been run by gene products (mRNA and proteins) deposited by the mother when she formed the egg. Not until the blastula contains some 4,000 cells is there any transcription of zygote genes. In fact, because the cells of the blastula are so small, the blastula looks just like the original egg to the unaided eye. Figure 14.2.4 Frog Bastulaĭuring this entire process there has been no growth of the embryo. A fluid-filled cavity, the blastocoel, forms within it. By the next day, continued cleavage has produced a hollow ball of thousands of cells called the blastula. However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus become smaller and more numerous. The next few cleavages also proceed in synchrony, producing a 16-cell and then a 32-cell embryo. Figure 14.2.3 Various stages of cleavage in a frog zygote The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal pole. The cleavage furrow again runs through the poles but at right angles to the first furrow. The second cleavage forms the 4-cell stage. This divides the egg into two halves forming the 2-cell stage. A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. The first cleavage occurs shortly after the zygote nucleus forms. ![]() The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells. ![]()
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