Blastula

The blastula is an early stage of embryonic development in animals. It is also called blastosphere. It is produced by cleavage of a fertilized ovum and consists of a spherical layer of around 128 cells surrounding a central fluid-filled    cavity called the blastocoel. The blastula follows the morula and precedes the gastrula in the developmental sequence.

Research
A whitefish blastula is often used to help study the processes of mitosis in animal cells. The blastula is prevented from multiplying during the nuclear-injection stage.

Formation of blastocyst
In mammals, blastulation leads to the formation of the blastocyst, which must not be confused with the blastula; even though they are similar in structure, their cells have different fates. The blastocyst consists of two primary cell lines:


 * the inner cell mass, also known as the "embryoblast"
 * the trophoblast

The former is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The latter combines with the maternal endometrium to form the placenta in eutherian mammals.