Problem 44

Background and context

After fertilization of male and female gametes, a single-celled zygote is formed. There are different stages of development of the embryo. The zygote undergoes rapid cell division many times (cleavage) to form an embryo of about 100 cells called the blastula.  It then undergoes a process called gastrulation in which the cells rearrange themselves into three germ layers. Each germ layer develops to form different tissues and organ systems. Organs are formed after gastrulation through the process of differentiation called organogenesis. 

The ectoderm forms the neural tube, and the mesoderm forms the notochord, which defines the central axis in the animal.