Cell Number in Haploid, Diploid and Polyploid Mouse Embryos

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Abstract
1. Cell number, determined by counting the nuclei, has been studied in preimplantation haploid, diploid, triploid, tetraploid and hexaploid eggs of Mus musculus, recovered usually 3½ days after fertilization. Most of the data concern triploid and tetraploid eggs, from the early morula to late blastula stages. 2. The mean number of cells in polyploid eggs (and perhaps in haploids), relative to the number in diploids of the same age, is approximately in inverse proportion to the number of chromosome sets present. Thus, the ratio of cell number in polyploid eggs to cell number in diploid eggs decreases with an increase in the number of chromosome sets. 3. There was no evidence for any one type of heteroploidy that the differences in ratios encountered from mouse to mouse were due to anything other than sampling error, i.e. the ratios remained fairly constant from the early morula to the late blastula. This suggests that the lesser number of cells in polyploids is already apparent in early cleavage. 4. These findings are compared with the parallel situation in Amphibia. 5. The counting of nuclei can play a role, but only a limited one, in the identification of polyploid eggs.