What is the embryo and how is ti formed?

Each embryo starts development as a zygote, a single cell resulting from the fusion of gametes.

Explanation:

the cells in a blastula-stage embryo start rearranging themselves into layers in a process called gastrulation.

the fossil record provides evidence of a common ancestor to many species.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds