The optimal oxygen tension for development of preimplantation mouse embryos to the blastocyst stage in vitro was found to be between 2.5% and 5%. One‐ and two‐cell embryos had a more sharply defined range of oxygen tension capable of supporting development than 8‐cell and morula stages. At all stages of development, more embryos developed to the blastocyst stage under 5% O₂ compared to the numbers of developing under higher oxygen tensions (20% and 40% O₂). The blastocysts developing under 20% O₂ had fewer blastomeres than those which developed under 5% O₂. As the time required for development to the blastocyst stage in vitro increased, there were fewer blastomeres present at the blastocyst stage.

These results indicate that the cleaving mouse embryo has an optimal oxygen requirement in vitro of about 5%. At higher oxygen tensions, fewer embryos develop to the blastocyst stage and in those which do develop, there are fewer cell divisions. If a gradient of oxygen tension exists across the blastomeres from the outside of the embryo to its centre, the blastomeres might be using this gradient to obtain information about their location within the embryo and respond accordingly. Thus blastomeres on the outside at a higher oxygen tension would divide at a slower rate and form trophectoderm whereas those on the inside at a lower oxygen tension would divide more rapidly and contribute to the inner cell mass.