As "the threshold gears are peculiar to all processes which are flowing past in the world", the problem of looking up of quantitative criterion determining transition of a biological system from one condition to another, is one of the main problems of natural sciences. For analysis of the given problem we investigated some stages of forwardness of marine aster Asterias rubens (mean and late blastula, early and late gastrula, early larva), dis- tinguishing among themselves by morphological characters. Change of structure of cell populations at each stage of development A. rubens analyzed with the help of method of quantitative assessment of functional condition of cells genome. The method is based on cytophotometryc analysis of cell populations coloured by Feulgen (quantifying of DNA in cell) and Naftol yellow S (quantifying of histones in a cell). As the criterion of estimation of functional condition of the cell was used the parameter CFAGEN, coefficient of functional activity of cell genome deduced from histone/DNA ratio after estimation of the absorbency of the nucleus coloured by two chromophores. The data obtained showed that at the moment of organism transition from one stage of development (late blastula) to another (early gastrula) the difference in CFAGEN values between these stages was statistically authentic. Thus comparison of cell distribution histograms has show that cell population conforming to the early gastrula contains 33 % cells with the level of genome functional activity that differs from this one in the cell population conforming to the late blastula. Just in the same moment, the transition of an organism from the stage of late gastrula to the stage of larva is accompanied by statistically authentic distinction in CFAGEN between these stages and availability of more than 1/3 cells (34 %) of the population at the larva stage showing genome functional activity different from that at the late gastrula stage. Thus, it is established, that the biological system passes to qualitatively new condition, if quantity of members of this system changes (increaser or decreases comparatively to a standard point of reference: the norm, previous system status and etc.) not less than on 1/3.