Describe the process of development of …

The embryonic stage of human development lasts from the time of implantation of the blastocyst in the uterus (around the end of the first week after fertilization) until the end of the eighth week after fertilization. ... In this process, the two-layered embryonic disc develops a third cell layer and a primitive gut. Embryonic development starts with the fertilization of the egg cell (ovum) by a sperm cell, (spermatozoon). ... The zygote undergoes mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of a multicellular embryo. For a lot of people, the first sign of pregnancy is a missed period. ... Other early pregnancy symptoms include feeling tired, feeling bloated, peeing more than usual, mood swings, nausea, and tender or swollen breasts. Not everyone has all of these symptoms, but it's common to have at least 1 of them. The heart is the first organ to form during development of the body. When an embryo is made up of only a very few cells, each cell can get the nutrients it needs directly from its surroundings.