Animal development:

When sperm fertilises the egg they fuse their information and form a zygote which is diploid.

This zygote undergoes mitosis until it creates a solid ball of 32 cells called a morula

As more cells are created, they secrete a fluid that forms a space in the centre resulting in a hollow sphere of cells called a blastula

The digestive tract is the first thing formed during the development of an animal

Gastrulation:

The process of reorganising the blastula into a 2 layered or 3 layered embryo

Indentation starts to form at a point on the blastula.This place starts to fold in on it self and is called the blastopore.

For animals which have the same spot for mouth and anus, the development stops here

For animals with distinct mouth and anus, the invagination continues until it reaches the other side and opens.

This structure of cells is called a gastrula {after the invagination stops in both the cases}

If the mouth is formed by the blastopore then that animal is called a protostome
If the anus was formed by the blastopore then the animal is called a deuterostome

Chordates(all vertebrates and a few other animals) are deuterostomes

Animals that have the same mouth and anus develop into 2 layers called the ectoderm and endoderm and are called diploblastic.
These layers are called the germ layers

Animals with distinct mouth and anus develop into 3 layers called the endoderm, ectoderm and the mesoderm and are called triploblastic.
The ectoderm develops into the skin, endoderm develops into the digestive tract and the mesoderm develops into everything else