How can you distinguish between the blastocyst and the gastrula stage?

Hint: During the development of a foetus it goes through many sequential stages having different structure and morphology and each structure has its corresponding names and morphology which differentiates one cell from another.

Complete answer:
This answer is consisted of two parts blastocyst and gastrula-
• As fertilization i.e. the fusion of sperm and ova takes place a zygote is formed. The zygote multiplies further giving rise to the embryo.
• The development of a foetus is constituted by constant cell division each; with each stage having its own name and finally an offspring is born. This process is called cleavage.

Blastocyst:
• As the zygote starts multiplying it gives rise to a ball of cells called the blastula which encloses a fluid filled cavity called the blastocoel.
• The blastula divides further to form the blastocyst which has an inner cell mass and an outer covering called trophoblast.

Gastrula:
• The blastula multiplies rapidly and by the process of gastrulation it folds upon itself and gives rise to a gastrula which is made of three layers of cells called embryonic germ cell layer namely-ectoderm, mesoderm and ectoderm.
• These further differentiate into other tissues and organs of the body. These layers give rise to different kinds of organs and organ systems.

Note: The rapid multiplication of zygote to form a mass of cells called embryos is called cleavage. Each stage in a cleavage has their respective morphology and name. The number of cells goes on increasing exponentially and finally a foetus or offspring is formed.
The formation of gastrula from blastocyst is called gastrulation. The blastocyst has an inner cell mass and trophoblast whereas a gastrula develops into three embryonic germ cell layers.