By aggregating two cells or two embryos, which carry distinguishable markers, to make chimaeras, it has been possible to investigate the cell distribution to the different tissues in pre-implantation embryos or post-implantation conceptuses. In this thesis, several series of chimaeras were produced to give further insights into the effects of cell number, embryo age, cell size, ploidy and size regulation on the contributions of two different cell populations to the various cell lineages in mouse chimaeric embryos. The first study used electrophoretic variants of glucose phosphate isomerase (GPII-A and GPII-B) to quantify the contributions of the different cell populations in E12.5 chimaeric conceptuses. Five series of chimaeras were produced: B(8+8), B(8+¹/₂8), B(¹/₂8+8), B(8+4) and B(4+8), where "B" represents the "genotypically balanced" strain combination (AAF₁ x AAF₁) ↔ (BF₁ x TGB) and the numbers represent the embryo stage (e.g. ½ 8 indicates half of an 8-cell stage embryo). There was no significant difference in the composition of the epiblast derivatives analysed between series B(4+8) and B (¹/₂ 8+8), or between B(8+ ¹/₂8) and B(8+4), and the cell populations contributed in the proportions expected from their 2:1 ratios at aggregation. Thus, this failed to support previous claims that the more advanced embryos/blastomeres made more contribution to the inner cell mass (ICM). A β-globin transgenic marker was used in the second study to analyse cell allocation in chimaeric blastocysts. Two sets of chimaeras were made by aggregating one big diploid cell (produced by combination of micromanipulation and electrofusion of a 2-cell stage embryo) with one normal-sized cell from a 2-cell stage embryo. These experiments showed that the bigger, but not developmentally less advanced, cells contributed more to the trophectoderm (TE) lineages than the ICM at blastocyst stage.