The process of embryogenesis begins when an egg or ovum is fertilized by a sperm cell to form a zygote.
This zygote then undergoes mitotic division, a process that does not result in any significant growth but creates a multicellular cluster called a blastula. This process is referred to as cleavage.
Deuterostomes and protostomes
The blastula develops in one of two ways, which actually divides the whole animal kingdom in half. The blastula develops a pore at one end, called a blastopore. If that blastopore becomes the mouth of the animal, the animal is a protostome, and if it forms an anus, the animal is a deuterostome.
Protostomes are invertebrate animals such as worms, insects and molluscs while deuterostomes are vertebrates such as birds, reptiles, and humans.
The blastula continues to develop, eventually forming a structure called the gastrula. The gastrula then forms three germ cell layers, from which all of the body’s organs and tissues are eventually derived. From the innermost layer or endoderm, the digestive organs, lungs and bladder develop; the skeleton, blood vessels and muscles are derived from the middle layer or mesoderm and the outer layer or ectoderm gives rise to the nervous system, skin and hair.
Once the germ layers are formed, the process of organ development or organogenesis begins. In vertebrates, the fist stage is referred to as neurulation, which involves the formation of the neural tube through folding of the neural plate. The heart and somites also form. However, from this point onwards, there is no shared pattern in enbryogenesis between the different species of organisms.