Before the 8th week of pregnancy, the embryo develops, its organs are laid, and after this period the embryo has all the main organs, and then only their development takes place. The period up to 8 weeks is called embryonic, and after 8 weeks it is no longer an embryo, but a fetus, and a fetal period begins.
The initial stages of development of the human embryo
The initial stages of embryo development can be traced by the day. On the first day the egg in the fallopian tube meets the sperm and the first stage - fertilization takes place. And the next day the zygote stage begins - a cell that has 2 nuclei in its cavity with haploid sets of chromosomes, after the fusion of which a cell with one nucleus and a diploid chromosome set is formed.
A day after this, the cell begins to divide - the stage of morula or crushing begins, taking up to 4 days. Each cell is divided until a single layer ball of cells with a cavity inside the blastula is formed. From its cells in the future formed trophoblast (future placenta) and embryoblast (the future child).
Just by the 7th day the blastula enters the uterus cavity, where it starts to secrete the enzymes necessary for the onset of the next phase - embryo implantation , which lasts up to 2 days.
Embryo after implantation
Just implantation gives rise to the next stage of embryo development - gastrula. A single layer ball of embryoblast cells turns into a two-layer ball. The outer embryonic layer is called the ectoderm and gives rise to the epithelium of the skin and the organs of the nervous system. This is the phase of differentiation of the embryonic sheets.
From the outer layer (endoderm) in the future, all the epithelial covers of the internal organs of the fetus (stomach, intestine, bronchi and lungs), as well as the liver and pancreas. These two layers bend, forming bubbles (amniotic - the future amniotic fluid and yolk - first to feed the embryo, and then as a hemopoietic organ).
From this moment (which ends at the beginning of the 3rd week of pregnancy), the last phase of the development of the embryo - organogenesis - begins.
Shortly before this, the embryo curves, its ectoderm covers the embryo from the outside,
After 4 weeks, the internal organs of the fetus begin to be laid. On the 6th week, the rudiments of the limbs will appear, until the end of the 7th, the heart and its chambers are formed, until the formation of all internal organs, lungs, and genital organs terminates. By week 9, all the organs and systems were formed completely, and then only their differentiation will take place.