Once an egg is released from the ovary during ovulation, it meets with a sperm cell that was carried to it via the semen. These two gametes combine to form a zygote and this process is called fertilization. The zygote then begins to divide and becomes a blastula.

 

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. During the process of gastrulation, cells start to move to the interior of the blastula and the three germ cell layers are formed. The body’s tissue and organs are derived from these three layers as follows:

• The innermost layer is the endoderm, from which the digestive organs, lungs and bladder develop.
• The middle layer is the mesoderm, from which the skeleton, blood vessels and muscles develop.
• The outer layer is the ectoderm, from which the skin and nervous system develop.

At this stage, the embryo is called the gastrula. Depending on the animal involved, a combination of the following processes takes place to arrange the cells inside the embryo:

• Epiboly - One sheet of cells expands over other cells
• Ingression - Individual cells migrate into the embryo
• Invagination - A sheet of cells folds in on itself to create a mouth, anus, for example
• Delamination - One sheet of cells divides into two sheets
• Polar proliferation - Cells at the polar ends of the blastula/gastrula grow, particularly at the animal pole
• Transcription of embryonic rather than maternal RNA
• Cell differentiation means cells lose their totipotentiality.