[Pre-implantation development of mouse embryos following home- and heterogeneous crosses]

The preimplantation losses among the mouse embryos at the age of 3.5 days in different crosses consist of unfertilized eggs (2-6%) and abnormal embryos (2-25%). When crossing the BALB and CBA females with the BALB, CBA and C57BL males, a similar genotypic influence of both the mother and father on the frequency of abnormalities in the embryos was found which could be expressed by an inequality. C57BL greater than BALB greater than CBA. The embryos of the BALB AND CBA females differed by the mitotic index and the occurrence of embryos without mitoses. The hybrid BALB X C57BL embryos had reliably more cells than all other hybrids.