It is a certain stage in human development, kind of like a four-and-a-half year-old.
Gridlock -- It is a certain stage in human development, kind of like a four-and-a-half year-old.
A blastocyst is a preimplantation embryo of about 150 cells produced by cell division following fertilization. The blastocyst is a sphere made up of an outer layer of cells (the trophoblast), a fluid-filled cavity (the blastocoel), and a cluster of cells on the interior (the inner cell mass). The uterus accepts a blastocyst for implantation for a limited period of time (the receptive phase) and then becomes refractory. And unlike most four-and-a-half-year-old's, a large percentage of blastocysts fail to implant and are naturally discarded by the body.