what is a blastocyst?
The blastocyst is the second major stage of development, after the morula, for a newly fertilized individual once cell division has begun. Cells begin to differentiate, or become specialized, and have moved on to an outer/inner arrangement: the outer cells are called the trophoblast which will give rise to the placenta, and the inner are called the embryoblast (or inner cell mass) as they will give rise to the embryo.