Variation in dry mass and volume of nonfertilized oocytes and blastomeres of 1‐, 2‐ and 4‐celled mouse embryos

Nonfertilized oocytes and 1‐, 2‐ and 4‐celled embryos were obtained from superovulated mice. The dry mass of cells was measured with an interference microscope either through the zona pellucida or after its removal with pronase. Volume was calculated from measurement of the diameter of the cells. From the dry mass/volume ratio the concentration of dry substance was calculated. Fertilization caused a drastic reduction in dry mass and volume from 34.6 to 31.4 ng and 189,500 to 165,000 μm3, respectively, while the concentration of dry mass of ova increased from 18.3 to 19.0 g/100 cm3. One‐celled embryos obtained 17, 23 and 34 hours after HCG gradually reduced their dry mass from 31.4 to 27.5 ng whereas their volume remained unchanged at the level of about 165,000 μm3. The increase of concentration after fertilization is followed by its reduction to about 17.0 g/100 cm3 between 17 and 23 hours after HCG and later till 34 hours remains unchanged. Mean dry mass of the blastomeres of 2‐celled embryos is one half, whereas that of blastomeres of 4‐celled embryos is one fourth of the dry mass of 34 hours 1‐celled embryos. A pronounced diversity of both dry mass and volume of the blastomeres of the embryo after second cleavage was observed.