Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. A fourth "layer" consists of the germ cells that are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organogenesis. The germ layers form during gastrulation of the blastula. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers.
Simpler animals qualified as diploblastic, such as cnidaria (which includes jellyfish, corals and hydra), possess two germ layers. Simpler animals, such as sponges, contain no "true" tissues as such, but have mainly two layers: pinacoderm, choanoderm, and a sandwiched mesohyl where totimpotent archaeocytes roam, having an immune and digestive function. Triploblasts emerged within the Diploblasts.