The developing organism is particularly vulnerable during the because this is when the distinct organ systems are being formed.

The developing organism is particularly vulnerable during the embryonic period because this is when the distinct organ systems are being formed.
This period begins with the fertilization (penetration of the egg by the sperm) and lasts until the end of the 10th week of the pregnancy. Single cell called a zygote is formed during this period.After that the fetal development begins. And then comes the stage organogenesis. In this stage the organs are formed.