[Duration of the cell cycles and mitotic phases during the cleavage period of the salmon Salmo salar L].

The temporal, parameters of the cell cycle in the Atlantic salmon embryos were studied at the temperature 4.8 +/- 0.1 degrees from 10.5 hrs after fertilization (end of interphase of the 1st cleavage division) till 93 hrs (beginning of blastulation). The period of cleavage comprises 11 cell cycles. The II--V cell cycles equal each other by their duration, then the divisions accelerate and the VII--X cell cycles are shorter than the former by 10%, on the average. The duration of some phases (pro-, ana- and telophase) in different cycles proved to be variable and all mitotic phases varied by their relative duration from cycle to cycle; however, in all cell cycles of the cleavage period the mitosis occupied 2/3 of each cycle. The synchroneous character of the passage of cell cycle phases by all the cells is preserved only up to 8-blastomere stage. Beginning from the 16-blastomere stage when two cell layers appear along the animal--vegetative axis, the gradient is initiated: the animal layer cells in the cycle phases leave behind the vegetative layer cells which border the yolk.