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How are blastomeres arranged when a blastula forms?
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Animal development
The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.
When a blastula forms during animal development, the blastomeres move outward to create a centrally located, fluid-filled cavity known as the blastocoel.1
The arrangement of these cells depends on the type of cleavage:
- Holoblastic cleavage: The blastomeres become arranged in a layer, typically one to several cells thick, that surrounds the blastocoel, giving the embryo the appearance of a hollow ball. This outer layer of cells is referred to as the blastoderm.12
- Discoidal cleavage: Because the cells do not surround the entire embryo, they lie only at the animal pole. In this case, the blastomeres may be arranged as a saucer-shaped blastodisk that covers the blastocoel, which forms as a crevice between the cells and the yolk mass.1
Regardless of the specific pattern, the formation of the blastula marks the conclusion of the cleavage period, preparing the embryo for the subsequent process of gastrulation.13