A microinjection technique has been devised for labelling individual blastomeres of preimplantation mouse embryos with a marker drop of inert silicone fluid placed in the cytoplasm either at the periphery of the egg or at the interface between two blastomeres (i.e. centrally). The following observations were made on blastocysts which developed from injected eggs:
(1) All drops derived from peripheral injections (at two-cell to morula stage) were found exclusively in the trophoblast.
(2) Drops injected centrally at two- and four-cell stages have been found in both the trophoblast and inner cell mass.
(3) Peripheral labelling of one or both members of a pair of eggs (four-cell to morula) fused into a chimaeric blastocyst yielded markers in both trophoblast and inner cell mass.
No evidence was found of inherent bilateral symmetry or polarity in the cleaving egg. The results indicate that physico-chemical positional effects determine whether a cell will differentiate into trophoblast or inner cell mass.
The results are discussed in the light of current hypotheses relating to early embryonic differentiation in the mammal, and it is suggested that cleavage occurs without spatial disturbance of the cytoplasmic pattern of the egg so that its cortical region is converted directly to the outer cells of the morula and hence to the trophoblast.