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It was shown on the example of chick embryo that the number tau 0 (tau n/tau 0) can be recommended as a measure of biological time and, for this purpose, the duration of the minimal mitotic cycle during synchronous cleavage divisions should be determined (in minutes) in various avian species. Based on the preliminary data, one can propose the comparability and similarity of the temporal programs of gastrulation and somitogenesis in the chick embryo and embryos of some fish and amphibians.

The work reported in this paper constitutes the first attempt to apply the method of the dimensionless characteristic of development duration (Dettlaff and Dettlaff, Arch. Biol. (Liege), 1961, vol. 72, pp. 1-16) to insects, specifically Drosophila melanogaster and Apis mellifera.

A sharp cooling of the sevryuga females as well as their reservation at the spawning temperatures suppress the ability of oocytes coated by follicle membranes to mature under the effect of hypophyseal gonadotrophic hormones in vitro and in vivo. At the same time the oocytes from these females in both the types of experiments are able to mature in vitro in the Ringer solution with progesterone. The injection of triiodthyronine to cooled or reserved females restores the ability of their oocytes to mature under the effect of hypophyseal gonadotrophic hormones both in vivo and in vitro. Such oocytes in our experiments undergo normal development upon fertilization. The thyroid gland hormone (triiodthyronine) influences thus, the ability of follicle epithelium to respond to the effect of hypophyseal gonadotrophic hormones by stimulation of oocyte maturation; this influence of triiodthyronine appears to be indirect. Some practical aspects of this problem are discussed.

The influence of sharp temperature shift and proportioned keeping of the sevryuga females at a low temperature on the oocyte maturation and ovulation has been studied. The reaction of the follicles to similar influences was shown to differ at different stages of the period of maturation inertia: the follicles remained intact at the stages prior to the germinal vesicle (oocyte nucleus) breakdown, were damaged in a part of females at the prometa- and metaphase I and in the most females at the prometa- and metaphase II. The injection of triiodothyronine to the females subjected to cooling at different stages of the period of maturation inertia exerted no effect on the oocyte ovulation and the egg fertilizability.

A summary of studies on temporal characteristics of animal development performed during last 30 years using dimensionless unit as a parameter of development duration is presented. This approach allowed to introduce time as a parameter in comparative embryological studies. Thus the time of development became a subject of the embryological studies. Perspectives for the future work are defined.

The karyoplasm (the contents of germinal vesicle) of the Bufo viridis, Xenopus laevis, and Acipenser stellatus oocytes maturing under the influence of progesterone acquires the ability to induce the maturation (germinal vesicle breakdown) of the full grown oocytes, when injected into them. This ability arises in the karyoplasm earlier than in the cytoplasm and is preserved until the germinal vesicle breakdown.

The data of Valouch et al. (1971) on tau 0 values in the smooth newt at different temperatures are presented as a curve. Using this curve, the absolute (in hours and days) duration of successive developmental stages provided by Glaesner (1925) is transformed in a relative one, expressed in the number of tau 0 (tau n/tau 0).

The use of the term "commitment" by different authors was compared and the term itself was compared with the term "determination". Different authors understand the term "commitment" in different ways. It is proposed to preserve the terms "competence", "determination" (labile and stable) and "differentiation" in studies of normal development at stages preceding the appearance of organ rudiments in order to facilitate the use of the knowledge acquired by experimental embryology and to decipher these concepts at the molecular level. The meaning of the term "commitment" should be made more precise when describing the experimental results and also when assessing the results obtained by various authors and published in numerous papers and reviews.

There was studied the effect of cycloheximide on the changes in the X. laevis and A. stellatus oocytes maturing under the influence of active cytoplasm, taken from the maturing oocytes, and, in the control series, under the influence of progesterone. The suppression of protein synthesis in the X. laevis oocytes by means of cycloheximide by 90--95% did not prevent the amplification of the factor of disintegration of the germinal vesicle membrane after 4 and 5 transfers of the active cytoplasm through the cycloheximide-treated oocytes; no pseudogastrulation was observed in such oocytes. Under the similar suppression of protein synthesis in the A. stellatus oocytes, the germinal vesicle did not break down already after the first injection of active cytoplasm; such oocytes acquried, however, the ability of cortical reaction. The cycloheximide-treated oocytes of both the species did not mature under the effect of progesterone. The fully grown A. stellatus oocytes matured after the injection of active cytoplasm from the mature X. laevis oocytes.

The dependence of the duration of one mitotic cycle during synchronous cleavage divisions (tau o) on temperature was studied in Rana temporaria, R. arvalis, R. lessonae, R. ridibunda and the corresponding curves were plotted in both linear and semilogarithmic scale. The curves can be used to characterize the relative duration of development, as well as to estimate the limits of optimal temperatures and to reveal the temperature-temporal regularities of development in these species.