How can you distinguish between the blastocyst and the gastrula stage?
Blastocyst is an early embryonic stage in mammals (otherwise it is a blastula), which is followed by development of gastrula.
Blastocyst is a hollow ball like structure that develops by repeated cell division from zygote . It is superficially covered by a layer of cells, collectively termed trophoblast. Inside there is a hollow cavity termed blastocoel, and an inner cell mass.
Blastocyst undergoes a process called gastrulation which involves migration of embryonic cells and a gastrula will be formed. Process of gastrulation in mammal is a bit different from other related vertebrates.
Unlike blastocyst, gastrula is three layered, with cells distinguished in ectoderm, mesoderm and endoderm . These three layers are called germ layers, and differentiation of various tissues/organs can begin from three germ layers.