One of the three tissue layers in an embryo; namely, the middle layer between the outer layer (ectoderm) and inner layer (endoderm). Through embryonic development, it will produce many internal organs of the adult, e.g. muscles, spine, and circulatory system. More specifically, the mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells, and coelomocytes. Mesothelium lines coeloms (the main body cavity and is positioned inside the body to surround and contain the digestive tract and other organs). Mesoderm forms the muscles in a process known as myogenesis, septa (cross-wise partitions) and mesenteries (length-wise partitions); and forms part of the gonads (the rest being the gametes).