Answer: hippocampus
Explanation:
Hippocampus is a structure of brain that remains embedded inside the temporal lobe. It has a role in developing and learning skills. This is a vulnerable structure which can be damaged by the different stimuli.
If the hippocampus get damaged because of illnesses for example. Alzheimer disease, or due to accident then the person may experience the loss of memory and loss of ability to retain long term memories.
The given situation, is an example of damage caused to the hippocampus then the explicit memories will not develop at the age of 18 months.