Ontogenesis has two aspects – qualitative and quantitative. They correspond to two main components of development of all organisms:
1) growth is increasing of sizes;
2) differentiation is increasing of сomplexity by means of isolation of partes and appearance of every possible differences.
The both aspects of the development are inseparable interconnected. The considerable part of differentiation passes by means of uneven growth. Differentiative growth leads to division of body on the parts and may be called as segmentary growth. Differently say, the main mechanic of development of organism in its ontogenesis is intermittent, polyfocal growth of organism on its extent: centres of intensive growth alternate with intermediate parts, which grow slowly and narrow between isolated, increasing anlages of organs. Epitheliums form the main, primary organizers (proliferative epithelial anlages) of morfogenesis. Mesenchyma, its cells move from embryonic layers (epithelioformed stratums), is orientated on the epithelial anlages of organs (differentiative parts of embryonic layers) and is devided between isolated organic, epitheliomesenchymal anlages. Mesenchyma and its derivatives can form secondary organizers of morfogenesis. They modify growth of the primary organizers. The main types of epithelial
growth:
1) by stratum, it can roll up (embryonic layers and neurulation, surface epitheliums);
2) tree-form growth – tubes of glandular epithelium and vascular endothelium repeatedly branch, their branches introduce into surrounding tissues with division of organ on parts (new organs, lobes, lobules).
Epithelial tube of body growths and branches much slowly.