How many cells form the inner cell mass?
If most of the cells of the blastocyst form the trophoblast, then how many cells of the compacted embryo actually form the inner cell mass? Beatrice Mintz (1970) solved this problem by making chimeric mice, wherein two mouse embryos are fused together during early stages of development. She took two 4-celled embryos, one from homozygous black-furred parents and one from homozygous white-furred parents, and she placed them together so that the cells integrated to form one eight-cell embryo. This embryo underwent compaction and was implanted into the uterus a foster mother mouse (see p. 364). Each cell can become a part of the trophoblast or a part of the inner cell mass with equal frequency. Mintz performed this experiment hundreds of times. If only one cell formed the inner cell mass, then each mouse pup should be either all white or all black. 0% should be mixed. If the inner cell mass was composed of two cells, one-quarter of them should be white, one-quarter black and 50% should be
