All of the approximately 200 different types of body cells can be classified into four basic or primary tissue types: epithelial, connective, muscle, and nervous (Table 4.1). These basic tissue types are often described by their embryonic origin. The embryo is essentially a three-layered tubular structure (Fig. 4.17). The outer layer of the tube is called the ectoderm; the middle layer, the mesoderm; and the inner layer, the endoderm. All of the adult body tissues originate from these three cellular layers. Epithelium has its origin in all three embryonic layers, connective tissue and muscle develop mainly from the mesoderm, and nervous tissue develops from the ectoderm.