The dynamics of structural changes of the chondriome in the early development of the sea urchin Paracentrotus lividus was studied. Mature eggs and embryos at various stages of cleavage were used for quantitative and ultrastructural analysis based on computerized 3D reconstruction from serial ultrathin sections. The following structural transformations of the chondriome were shown to occur in the course of embryogenesis: (i) 15 min after fertilization, mitochondrial clusters disintegrate, and mitochondrial division is induced. At the stage of two blastomeres the population of mitochondria increases twofold; (ii) the mitochondria divide by means of the contraction of both outer and inner membranes. The forming furrow divides the "parental" mitochondrion into two equal "daughter" parts; (iii) at the four-cell stage the division ceases, and mitochondria start to grow, so that the mitochondrial length increases; (iv) cell differentiation further stimulates elongation of rod-shaped mitochondria, and the ratio of rod-shaped to spherical mitochondria changes; (v) in an unfertilised egg, the mitochondria are in a condensed form; after fertilisation all the mitochondria acquire a conventional form. Modern concepts of chondriome proliferation in eukaryotic cells are discussed.