The morphological description of morphogenesis accompanying primmorph formation from cell aggregates in the endemic Baikal sponge Lubomirskia baiсalensis as well as the description of cell composition of these primmorphs was investigated. Development of small sponges from dissociated cells in L. baiсalensis is a stepwise process that can be separated into three stages: 1 – primary aggregates; 2 – early-stage primmorphs; and 3 – primmorphs or young sponges. Primary aggregates are irregular in shape and consist of loosely packed cells. Early-stage primmorph development is characterized by cell sorting, aggregate compactization, surface epithelization and cell differentiation. Primmorphs or young sponges have developed elements of the aquiferous system. The main morphogenesis occurring during primmorph development in L. baiсalensis is the epithelization of the cell aggregate surface accompanied by the flattening of poorly connected spherical cells. The main sources of new exopinacoderm development during L. baiс