Answer:
Blastocyst
Step-by-step explanation:
As morula moves towards the uterine cavity by the end of the fourth day of fertilization, there is an increase in the number of its cells. Morula is transformed into a hollow ball of cells that has an internal cavity called blastocele, outer cells (trophoblast) and the inner cell mass.
This structure is called blastocyst which is present as a free structure in the uterine cavity for 2 days after which it becomes attached to the endometrium of the uterine wall. Attachment of blastocyst to the endometrium of the uterine wall is called implantation. The enzymes secreted by blastocysts make a burrow in the endometrium to facilitate implantation