1-Cleavage •Fertilization of the chick egg occurs in the oviduct, before the albumen and shell are secreted to cover it. •Like the egg of the zebrafish, the chick egg is telolecithal, with a small disc of cytoplasm—the blastodisc(zygote)—sitting atop a large yolk. •Like fish eggs, the yolky eggs of birds undergo discoidal meroblastic cleavage. •Cleavage […]
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Steps involve in activation of egg metabolism 1-Release of intracellular calcium ions •The release of Ca+2 that occurs when the sperm enters the egg is critical for activating the egg’ s metabolism and initiating development. •Calcium ions release the inhibitors from maternally stored messages, allowing these mRNAs to be translated •They also release the inhibition
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Introduction to hormonal control of metamorphosis •The control of metamorphosis by thyroid hormones was first demonstrated in 1912 by Gudernatsch, who discovered that tadpoles metamorphosed prematurely when fed powdered horse thyroid glands. •In a complementary study, Allen (1916) found that when he removed or destroyed the thyroid rudiment of early tadpoles (thyroidectomy), the larvae never
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Introduction Metamorphosis is the complete change of structures in larvae or egg form of organisms into an adult form regulated by many factors such as hormones. •In most animal species, however, embryonic development includes a larval stage with characteristics very different from those of the adult organism, which emerges only after a period of metamorphosis;
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Introduction to polyspermy The entrance of multiple sperm polyspermy leads to disastrous consequences in most organisms. In the sea urchin, fertilization by two sperm results in a triploid nucleus, in which each chromosome is represented three times rather than twice. Normally each sperm’s centriole divides to form the two poles of a mitotic apparatus, the
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Introduction of spermatogenesis Spermatogenesis is the process in which an animal produces spermatozoa from spermatogonial stem cells by way of mitosis and meiosis. Spermatozoa are the mature male gametes in many sexually reproducing organisms. Stages of Spermatogenesis The entire process of spermatogenesis can be broken up into several distinct stages, each corresponding to a particular type of cell in humans. In the following table, ploidy,
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Introduction All the material necessary for the beginning of growth and development must be stored in the egg, or ovum. Whereas the sperm eliminates most of its cytoplasm as it matures, the developing egg (called the oocyte before it reaches the stage of meiosis at which it is fertilized”) not only conserves the material it
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