Cleavage: zygote undergoes multiple cell division with no significant growth → cytoplasm is partitioned to several cells
• Formation of a blastula
Gastrulation: establishment of embryonic germ layers
• Formation of a gastrula
• Blastopore is differentiated → becomes either the mouth or anus
After fertilization, embryonic development progresses through three stages:
1. cleavage, during which the zygote develops into a blastula through extensive cell division;
2. gastrulation, during which three tissue layers are established by the dramatic rearrangement of cells, forming a gastrula;
3. organogenesis, during which the rudiments of organs develop.
The details of embryonic development vary by species, but the developmental sequence is consistent. Embryonic development has been studied extensively in a few species, including sea urchins, frogs, and chickens. All the images in this tutorial are based on frog development.
Determine which terms, phrases, and images describe each phase of development.
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