The beginning of ontogenesis in many living organisms, including flowering plants, as a rule, is embryogenesis. It is well known that embryogenesis is a single integrated process. At the same time, from the standpoint of the methodology of research on flowering plant embryogenesis it is proposed to distinguish different, although interrelated, stages of this process. Such stages reflect the pulsating nature of the development of the embryo as a dynamic system. The stages differ in duration, morphophysiological parameters, structural and functional characteristics. A number of authors regard some stages of flowering plant embryogenesis as critical, while offering various criteria for distinguishing such stages. This review briefly discusses those proposed criteria: (1) The increased sensitivity of the embryo to external and internal negative effects; (2) The change of the structural and functional characteristics of the embryo and the surrounding tissues of the seed; (3) The autonomy of the embryo; (4) The possibility of switching the embryo development program to alternative pathways. The issues concerning the morphophysiological features of embryos at the critical stages are analyzed.
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