When the sperm and the egg combine together, the resulting structure is called a zygote. When this zygote divides further into multiple cells shaped like a ball-like structure, it is known as a blastocyst.
Step-by-step explanation:
A zygote is a single-celled structure, which signifies the very first stage of life. It holds all the 46 chromosomes necessary for a human being. When fertilisation occurs, the egg cell combines with the sperm and results in this zygote.
Generally, this occurs in the fallopian tubes if fertilisation occurs within the body. However, it can also be carried out in a petridish under laboratory conditions outside the body.
A blastocyst is the same structure, but after a couple of days (~5 days in humans) after fertilisation. It has now transformed into a ball of cells with a small cavity inside the embryo, where the baby will eventually take shape.
The inside cells of the blastocyst develop into an embryo, while the outer cell layer will act as membranes for nutrition and protection. This blastocyst forms when the zygote travels down the fallopian tubes. When it reaches the uterus, it consequently buries itself in the uterine wall.