Gastrulation in Animals
All animals begin as a single celll, called a Zygote. As this Zygote cell develops and grows, it eventually splits into two cells, then four, then eight, then sixteen. All of these cells stick together in the form of a hollow ball, or Blastula. Eventually, the ball caves in and tucks into itself, by a process called Gastrulation, creating a nook called a Blastopore, turning the blastula into a Gastrula.