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. This cleavage is followed by a period of development in which the most significant events are movements of cells relative to one another. An adult, multicellular animal typically possesses a concentric arrangement of tissues of the body. These adult tissues are derived from three embryonic cell layers called germinal layers; the outer layer is the ectoderm, the middle layer is the mesoderm, and the innermost layer is the endoderm. Gastrulation involves the drastic reshuffling of the blastula's cells into these three germinal layers, which in turn become the various organ systems of the animal's body. Gastrulation consists of the inward movement, or invagination, of the cells of one side of the blastula until they are juxtaposed to the opposite side; thus the spherical embryo is converted into a double-walled cup, or gastrula. The involuted part of the blastula, lining the inside of the double-walled cup, gives rise to the endoderm and the mesoderm, and the cells remaining on the exterior of the cup become the ectoderm. From this ectoderm, or outside layer, will be derived the outer part of the animal's integument (the skin covering) and its nervous system and sense organs. The inward movement of one side of the blastula's cells has contracted or eliminated the blastocoel, which was the cavity of the blastula, but a new cavity is formed in the hollow of the cup. This primitive gut cavity is the endoderm and forms the rudiment of the animal's future gut and many of its associated digestive organs and glands. Before either ectoderm or endoderm begins to differentiate into their future structures, however, a third layer of cells lying between them becomes apparent. This third layer is the mesoderm, from which will later be derived the bulk of the animal's musculature and, in most cases, its excretory system and reproductive system. The diversification of cells in the embryo progresses rapidly during and after gastrulation. The visible effect is that the germinal layers become further subdivided into aggregations of cells that assume the rudimentary form of various organs and organ systems of the embryo. Thus the period of gastrulation is followed by the period of organ formation, or organogenesis.