Formalization of existing quantitative notions on module structure of tissues and, in particular, squamous epithelium++ has been performed. An idea has been developed on the elementary tissue unit (ETU), appearing as a consequence of divergence ++ of functions among cells. The tissue is considered as a result of polymerization of the ETU. According to cell composition in the ETU, the tissues are proposed to be divided into simple and complex ones. Certain sets of rules have been formulated concerning polymerization of the ETU, they allow to find a lot of all possible constructions of simple and complex tissues and, in this way, prognosticate their development in phylo- and ontogenesis. A part of theoretically found constructions are demonstrated to be realized in some animals. New informative signs are suggested for studying tissues in normal development and in pathological conditions.