AP BIOLOGY

Detailed explanation-1: -gastrula, early multicellular embryo, composed of two or more germinal layers of cells from which the various organs later derive. The gastrula develops from the hollow, single-layered ball of cells called a blastula which itself is the product of the repeated cell division, or cleavage, of a fertilized egg.

Detailed explanation-2: -During gastrulation, a hollow cluster of cells called a blastula reorganizes into two primary germ layers: an inner layer, called endoderm, and an outer layer, called ectoderm.

Detailed explanation-3: -Gastrulation is defined as an early developmental process in which an embryo transforms from a one-dimensional layer of epithelial cells (blastula) and reorganizes into a multilayered and multidimensional structure called the gastrula.

Detailed explanation-4: -Animals develop either two or three embryonic germs layers (Figure). The animals that display radial symmetry develop two germ layers, an inner layer (endoderm) and an outer layer (ectoderm). These animals are called diploblasts. Diploblasts have a non-living layer between the endoderm and ectoderm.

Detailed explanation-5: -Ancestral metazoans such as the Cnidaria have only two cell layers, an ectoderm and an endoderm. Following Thomas Henry Huxley (1871), the two layers of adult Cnidaria are homologous to two of the germ layers in vertebrate embryos-ectoderm and endoderm, i.e., they are conserved across the eumetazoa.