Answer:
A sphere of cells surrounding a hollow center
Explanation:
When the sperm penetrates the egg, an embryo formation process begins, which will develop during pregnancy. The embryo goes through several stages, the blastocyst is one of them.
After the fertilized egg, a diploid cell called a zygote is formed. Some time later, cleavages begin, which are successive cell divisions. In the first (cleavage), the zygote divides into two cells called blastomers, and the cleavage process is repeated, increasing the number of cells to form a sphere called morula. The morula is formed in the fallopian tube, which drives the embryo toward the uterus, while cleavages continue until it reaches the uterus when it receives uterine fluids. On the 3rd or 4th day of development, some liquid filled spaces appear.
The blastocyst phase begins at approximately the 5th day of embryo development, when the cells form the trophoblast, embryoblast and blastocele. The trophoblast contributes to the formation of the placenta, and the embryoblast will contribute to generate the embryo.
In this process, the internal mass of the blastocyst is formed. When embryonic development reaches the blastocyst stage, it is very close to complete embryo formation, ideal for implantation into the womb.
A blastocyst is an embryo 5 or 6 days old (in some cases 7 days) and is composed of approximately 200 cells. Compared to embryos with fewer days, this embryo has a more differentiated cell structure, composed of a central cavity (blastocele cavity) and two cell types: the trophoectoderm cells (which later will develop and form the placenta) and the cells of the internal cell mass (which will form the fetus).
