Regularities of early embryogenesis in teleosts as revealed by studies of the temporal pattern of development : I. The duration of the mitotic cycle an its phases during synchronous cleavage divisions.

The rate of the first four cleavage divisions has been investigated in 8 teleost species. A comparison of cytological studies of the duration of the mitotic cycle in four species with visual observations on the intervals between the appearance of furrows of the first four cleavage divisions in the same species at the same temperatures has shown that in teleosts the duration of the mitotic cycle during synchronous cleavage divisions ({ie301-1} equals one half of the time period between the appearance of furrows of the 2nd and 4th cleavage divisions. At optimal temperatures, it also corresponds to 1/5 of the period between insemination and the appearance of the 1st cleavage furrow. Using these two method, {ie301-2} values were calculated in the species under study for a wide temperature range and curves of the temperature-dependence of {ie301-3} were constructed. Temperatures below the spawning temperature range produced a shortening of the relative duration (in terms of {ie301-4} units) of the period between insemination and the appearance of the 1st cleavage division furrow, as well as a shift in temporal relationships between the successive mitotic phases: the relative duration of the active mitotic phases (especially metaphase) decreased sharply while the overall relative duration of telo-, inter-and prophase increased correspondingly. On the basis of the results obtained the question of the determination of the lower limit of the optimal temperature range in teleosts is discussed.