According to Edwards and Hansen [27], there are at least two possibilities to explain
why embryos become more thermotolerant as development proceeds. One possibility is
that a larger embryo with more cells is better able to survive the loss of a fraction of its cells
than embryos with a smaller number of cells. If one assumes hypothetically that the effect
of heat shock is to alter the function of 50% of the blastomeres, a two-cell embryo would be
left with only one blastomere to form a viable embryo, whereas a morula or blastocyst
would have 30–50 viable blastomeres to continue development. The second possibility is
that embryos acquire mechanisms of thermo-protection during development, such as
production of heat shock proteins and glutathione