What is a bastocyst?

Believe you meant "blastocyst". Defined as a distinctive stage of a mammalian embryo. It is a form of blastula that develops from a berry-like cluster of cells, the morula. A cavity appears in the morula between the cells of the inner cell mass and the enveloping layer. This cavity becomes filled with fluid. The blastocyst differs from the blastula in that it is composed of two already differentiated cell types, the inner cell mass and the enveloping layer.