Early Development. Chapter 21. Early Development. Highly variable among different organisms Common genetic and cellular mechanisms in development Begins with gametogenesis Proceeds in ordered phases. Phases of Human Development. Gametogenesis and Fertilization. Gametogenesis.
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embryo–is shown here.
In most animals, cleavage results in the
formation of a multicellular stage called a blastula.
The blastula of many animals is a hollow ball of cells.
The zygote of an animal undergoes a succession of mitotic cell divisions called cleavage.
The endoderm of
the archenteron de-
velops into the tissue
lining the animal’s
Cross section of blastula
The blind pouch
formed by gastru-
opens to the outside
via the blastopore.
Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the blastocoel, producing layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer).