I do surgical embryo transfer of genetically modified (microinjected) mouse embryos (at 1 or 2 cell stage). As most of the constructs are new, we expect that it may affect birth rate. In case there are no or very low number of pups born we try to investigate at what stage the problem occurs. I dissect the embryo recipients on day 21(+48 hrs to make sure that we don't sacrifice a mouse that is late to litter down). Sometimes I find dark, gelly, regularly interspaced structures on the mesenterial side of the uterus. Initially, I thought them to be implantation sites of the embryos that have been subsequently resorbed for some reason. Until I found similar structures in mice that haven't been mated with male. My question is: are there anatomically predefined sites in an uterus that embryos can invade? Are my dissections just a waste of time and yield no valuable data?