Which of the following events happens first in the development of a new organism?

a. A morula is formed.

b. A blastocyst implants into the uterine wall.

c. A sperm fertilizes an egg to form a zygote.

d. A zygote undergoes mitosis.

The answer is c. A sperm fertilizes an egg to form a zygote.

The first step in the development of a new organism is a sperm fusion with an egg and formation of a zygote. The zygote then undergoes mitotic divisions and as a result a morula forms. Morula is 16-cell mass that continues to divide and forms a blastocyst, which later implants into the uterine wall.