Male mice, heterozygous for the T26H reciprocal translocation, were mated to normal females. A systematic histological study at day 6 and day 8 of pregnancy, covering altogether 303 implantation chambers, yielded the following results: the average number of decidual reactions per female was 8·91, the average number of morphologically normal embryos atday 6 was 4·61 and at day 8, 4·25. No significant embryonic loss occurred between day 8 and the end of pregnancy. Most (89·0 %) of the embryonic loss due to genetically unbalanced (deficiencies combined with duplications) progeny occurs between the moment of the induction of a decidual reaction and day 6, i.e. in ±1·5 days. When the genetically unbalanced embryos were grouped in three classes according to the size of their deficiencies, the number sand types of the aberrant histological pictures found could be reconciled with the expectations of the three classes, based on cytological analysis. According to this, embryos with the smallest deficiencies take just over 2 days before all have died. Embryos with medium-sized deficiencies take ±1–5 days, and the group with the largest deficiencies take just over 1 day. At day 6, 8·8 % of the implantation chambers had an intact epithelium whereas atday 8, this had dropped to 1·3 %. This is consistent with the view that erosion of the uterine epithelium at the site of implantation is an autonomous process, but helped by the phagocyti caction of the trophoblast cells.

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