Ovulation
Every month an adult woman releases one egg or ovum from one of the two ovaries. Usually, the egg is released on 14 days of the month. This process is called ovulation. So the development first step is ovulation.
Fertilization
The egg released from the ovary is caught by finger-like projections of a fallopian tube called fimbriates. The egg enters the fallopian line. Millions of sperms from vigina to uterus travel towards the egg in the fallopian box. From thousand of sperm, only one sperm succeeds infused with an egg. So the fusion of egg and sperm occurs in the fallopian tube, this process is called fertilization, and the structure formed after fertilization is called a zygote.
Cleavage.
After fertilization, through mitosis, the zygote starts dividing into several cells. This process is called cleavage. The first cleavage is vertical, where the zygote is divided into two compartments. The second cleavage is horizontal, where cells number increases into four. The third cleavage results in eight cells, and in the fourth cleavage, sixteen cells are formed. These 4 steps completes the stage of cleavage.
Morula
After the fourth cleavage, the sixteen cells stage is called a morula, where several cells start rearranging themselves. The cells of the morula stage are called blastomere.
Blastula
In the blastula stage, the cells rearrange themselves. Some of the cells arrange themselves outside, and some remain inside. The outside cell is called "outer cell mass" that becomes trophoblast and helps form the placenta. The inside compartments are called "inner cell mass," and the inner cell mass becomes embryoblast, allowing embryo formation. A hollow cavity called blastocoel is also formed in the blastula stage.
Gratulation
In the Gastrulation stage, the"inner cell mass" called embryoblast, cells start arranging themselves into germ three layers. First, two layers of cells are formed; epiblast and hypoblast. The blastocoel cavity also divides into two holes, One is called the yolk sac, and the second is the amniotic cavity.
Some of the cells of epiblast star migrating upward and form a rod-like structure called primitive streak. The upper pointed portion of the primitive streak is called the primitive node. The cells between primitive streak and primitive node invaginated start moving on sides. So the inside of the primitive streak becomes empty.
The primitive streak changes into a hollow structure called primitive groove, and the upper part changes to a primitive pet or Hanson node.
Some of the cells of epiblast start migrating downside and form another layer of cells in the middle. Some cells push hypoblast cells and arrange themselves and form a layer that is later called endoderm. The upper epiblast layer becomes the ectoderm, and the middle layer of the cell is called the mesoderm.
Neurulation
In this stage, the ectodermal layer forms a structure called a neural tube that includes the nervous system later in the future. The mesodermal layer starts forming a tube-like Structure called the notochord, which forms the vertebral column in the end. The endoderm is from a structure called archenteron. It is the gastric cavity that forms the gut in the future.
The Next stage is organogenesis, where the formation of organs starts from three germ layers.
The Ectodermal layer forms the Skin and nervous system. The mesodermal layer includes the muscles and skeletal system, and the Endodermal layer comprises the Digestive and respiratory systems.
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