Regularities of early embryogenesis in teleosts as revealed by studies of the temporal pattern of development : II. Relative duration of corresponding periods of development in different species.

The absolute durations in minutes of the periods from insemination till the onset of gastrulation, the end of epiboly and appearance of the 10th pair of somites, as well as the intervals between these stages, have been determined in 7 species of Teleostei. Their relative duration, in terms of mitotic cycle time units {ie313-1} has also been determined. The biological age (expressed in number of {ie313-2} units) at which the isolated blastoderm becomes capable of differentiating in vitro was also determined in 4 of the species.The relative duration of the periods studied in all the species was shown to be stable at optimal temperatures. The corresponding periods of development have similar duration in closely related teleost species (i.e. belonging to the same genus).The relative duration of the cleavage period varies among the species belonging to different families. These variations are correlated with differences in the amount of yolk in the egg and the degree of psychrophily of the species compared. By contrast, the relative duration of the period between the onset of gastrulation and the stage of 10 pairs of somites was found to be very similar in species which belong to different orders and differ with respect to the amount of yolk in the egg and the temperature ranges of development. The rate of movement of blastoderm cells during epiboly is higher, the larger the eggs in the species under study. The variations in the age at which the isolated blastoderm acquires the ability to differentiate in vitro are not always correlated with the amount of yolk in the egg.The hypothesis is proposed that an interaction between the yolk and active cytoplasm, which determines time of the onset of ribosomal RNA synthesis, is of importance in controlling the temporal relationships in early embryogenesis.