The time period during which an organ is coming ""on-line"" during pre-natal development is referred to as the:

Embryonic period

Explanation:

During Prenatal development, the development of an organism takes place in three stages  

1)Stage I – When the organism is at the stage I i.e the germinal stage a zygote is formed after a sperm fuses with an egg. The mass of cell move through the fallopian tube and gets embedded in the uterus wall. This process is called implantation.  

2)Stage II – In the second stage i.e Embryonic Stage major organs start developing and come in line. After this stage, the embryo is one inch long.  

3)Fetal stage – Sex organ, bone, muscle start forming.