Ablastocyst is

cluster of cells surrounding a developing egg in the uterus.

membrane that surrounds and protects a growing embryo in the uterus.

growth that can form in the wall of the fallopian tubes.

hollow ball of cells formed after a fertilized cell undergoes cell division.

hollow ball of cells formed after a fertilized cell undergoes cell division.  

A blastocyst is a thin walled hollow structure that succeeds the zyotic development in some animals and humans. It consists of cluster of cells called as inner cell mass from where the embryo forms. The outer layer cells of the blastocyst form placental cells and other supporting tissues that are required for fetal development inside the uterus. These cells undergoes rapid cell division to develop into specialized cells in the fetus.

A blastocyst is a hollow ball of cells formed after a fertilized cell undergoes cell division.

Explanation:  

After fertilization, the zygote divides to form a ball of cells called as morula. Morula divides further to form blastula or blastocyst in mammals. Unlike morula, blastocyst has a cavity called as blastocoele. Blastocyst further divides to form gastrula in which the germ layers of embryo are formed.  

Further Explanation:

Morula is a ball of 16 cells formed after cleavage in zygote. When around 100 cells have been formed, a hollow cavity is finally produced called as blastocoel and the entire structure is known as blastocyst in mammals. Blastocyst has a layer of cells called as blastoderm which lines the blastocoele.

Blastocyst is divided into two parts

In blastula, cell polarity and axis specification is established. Activity also occurs to regulate gene expression. During mid blastula transition maternal genes are degraded and fetal genes finally undertake the process of gene expression.

Learn More:  

about blastocyst:  ( answer by TenishaTon )

about embryonic development:  ( answer by Bistai )

Keywords:

blastula, blastocyst, blastocoele, morula, inner cell mass, trophoblast, mid blastula transition