Fig. 1. Clones derived from the two-cell stage tend to occupy either embryonic or abembryonic parts of the blastocyst. Blastomeres of two-cell embryos were labelled with DiI (red) or DiD (blue) and the distribution of the progeny of labelled cells were analysed at the blastocyst stage. The frequencies of the four categories of blastocyst scored are indicated below. A total of 69 blastocysts were classified according to the extent to which cells derived from one blastomere comprising mainly the embryonic part (A) crossed the embryonic-abembryonic boundary zone (consisting of a region one cell deep and parallel to the roof of the blastocyst cavity, i.e. between broken white lines) into the abembryonic part and (B) vice versa. Comparison of the data in the row A and row B indicated that cells from the abembryonic part showed a greater tendency (² P<0.02, 3 d.f.) to contribute to the embryonic part than their embryonic cousins to contribute to abembryonic part. Blastocysts were scored ++ if up to two cells crossed the boundary zone (arrow). In cases where three cells crossed this boundary, blastocysts were scored +. When four cells, or five or more cells crossed the boundary the blastocyst were scored – and ––, respectively. The micrographs represent individual optical sections mid-way through the embryo to show the cavity, which occupies the lower half of each blastocyst (see Materials and Methods). The broken yellow lines show the border between clones derived from each blastomere. Scale bar: 25 µm.