Answer
The blastocyst (Latin --germ bag) forms from the morula when the latter is about 32 cells large. About this time, fluid from the uterine cavity, passes between the cells of the morula and settles in the midlldle of the structure. The effect is to form a fluid filled cavity in the erstwhile morula which now transforms into a blatocyst --with a fluid-filled cavity (blastocyst) and a double layer of peripheral blastomeres. The surrounding blastomeres then differentiate into two parts--the outer cells or epiblast, and the inner cell mass or embryoblast. The inner cell mass will go on to form the embryo. The epiblast will give rise to the trophoblast-- the blastocyst at this stage is about 100 cells cells large. The trophoblast ( "tropho-" grow or nourish) secretes fluids that will nourish the embryoblast ; it will also give rise to the outer chorionic membrane and the fetal part of the placenta.
Work Step by Step
From the cells of the embryoblast, the structures of the embryo proper will develop. By about the fifth day after fertilization, the blastocysts escapes the zona pellucida and lies free in the uterine cavity, for about two days. After this period, attachment to the uterine wall ( in fundus or body) takes place, and implantation proper follows shortly.
