II. A. Multiple Choice. Pick out the letter of the correct answer. 11. When the spem cell and egg cell are joined together to form a unit, it is called a. embryo b. fertilization c. reproduction d. zygote 12. Another term used for sex cells is a. embryo c. ovary b.gametes d. zygotes 13. The time between the laying of egg until its hatching of the new one is called a. incubation c. pregnancy​