Oviductal Recognition of Embryonic Signals

In most mammals studied, the embryo starts developing in the oviduct and about the time it enters the uterus it is constituted by a few blastomeres rarely exceeding 12 to 16 in number. It is hard to believe at first sight that such minute biologic entity may have to signal its presence to insure its survival at the same time that its few cells are so busy executing a progressively complex developmental process.