Spatial distribution of yolk platelets and fat droplets in oocytes and cleaving embryos of the common frog (Rana temporaria) and toad (Bufo bufo).

Animal-vegetal gradients of fat droplets and yolk platelets have been quantitatively determined in mature oocytes and in cleaving embryos in two anuran species Rana temporaria and Bufo bufo using stereological methods. Volume densities Vv which describe properly the amount of nutrients in embryos have been used during the measurements. The embryos till the stage of late blastula are spherical: the spherical form is not distorted even by blastocoel forming eccentrically, in the animal hemisphere. Increased diameters during cleavage, which have been observed in the two species, can be ascribed to growth of the blastocoel and increased number of blastomeres while the quantity of the cytoplasm does not increase. The volume densities of yolk platelets and fat droplets per unit of cytoplasm volume remains constant throughout the cleavage. Volume densities change along the animal-vegetal axis in accordance with the course of the 3rd polynomial depending on the distance of the area under study from the vegetative pole f: y-->Vv, f = ay3 + by2 + cy + d. Parameters of fitting functions a, b, c and d change considerably during development of the embryo, which proves reorganisation of the cytoplasm during cleavage: the most significant changes occur from fertilisation to the four-cell stage and during growth of the blastocoel from morula to late blastula stage. Distortion of the axial symmetry of yolk platelet distribution in embryos occurring after fertilisation is not extensive in the two studied species. The mathematical model of spatial distribution of yolk platelets and fat droplets in oocytes and cleaving embryos in the studied anuran species was proposed using the above data.