biological clock in vertebrate embryogenesis as a mechanism of general control over the developmental organism

The problem of understanding the role of the time factor in embryogenesis is still at the conceptual stage. At the same time, a number of rhythmic processes described in the embryogenesis of animals point to the involvement of a biological clock in this period of ontogenesis. Most of them (biochemical, biophysical, cytological) have been identified during the process of cleavage and have a duration equal to that of a single cleavage division τ₀. The current paper considers mainly the development of salmon fish embryos, which, in comparison with the widely used models (Danio rerio, Orysias latipes), have some extra advantages: they are about twice as big at similar stages and develop 30-40 times slower. The most pronounced and rhythmical process in vertebrates is somitogenesis. The work presents a series of fundamental facts in relation to the temporal and spatial aspects of somitogenesis: 1) the formation of each new somite occurs within a constant time τ S ; 2) this time is controlled to a high degree of accuracy; 3) the size of new somites (l S ) along the anterioposterior axis is constant; 4) the temporal factor, in contrast to the spatial, plays the primary role during morphogenesis of somites. The rhythms τ₀ and τ S in different species are equal or are multiples of each other, an indication of their common origin. A high degree of synchronism in embryo development from the start of cleavage to the end of somitogenesis is revealed. This proves the existence of constant temporal control of their development during most of the period of embryogenesis. It is proposed that the endogenic rhythms under discussion are responsible for the coordination and integration of multilevel processes in embryogenesis, compensating for the lack of a formed nervous system during this period.