Formation Of Germ Cells

In some species, distinctive granular inclusions can be found in the posterior cytoplasm of the egg. The cells that inherit these granules become the germ cells and eventually migrate into the ovaries or testes to become sperm and eggs. When the germ cells are ablated, the germline is missing and the individual is sterile, as noted in 1911 by Hegner. The nuclear energids (in some species, such as Drosophila) arrive at the posterior before reaching any other egg region, and the cells that will include this specialized cytoplasm cellularize earlier than any of the other cells. Germ cells rarely grow or divide during embryogenesis. The early segregation of these cells is thought to protect them from potential errors incurred during division and differentiation that might damage the genetic material necessary to build the next generation. In other species (e.g., most Lepidoptera), segregation of the germline occurs in the middle of the blastoderm; in other species no apparent germ cells can be detected at the blastoderm stage.

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