A New Factor Associated With the Implantation of the Mouse Egg

A feature of apparent significance in the implantation of the blastocyst of the mouse, which does not appear to have been described, was observed in a series of normal pregnancies. The pregnant uteri were obtained from females killed at intervals of 2 hr from 72 to 120 hr after timed matings. The uteri were fixed in aqueous Bouin, in Zenker or in Carnoy's fluid. Sections were cut in paraffin wax at 7 to 15 μ and stained with haematoxylin and eosin. Shortly after entering the uterus at about 72 hr post coitum, the blastocysts become randomly distributed throughout the uterine lumen. Between 84 and 96 hr post coitum they become antimesometrially situated in the lumen, then they lose their zona pellucidae and become orientated with their embryonic poles directed mesometrially. When the blastocysts reach their implantation sites, certain cells with a quite distinct morphology can be seen within the inner-cell-mass.