Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Arachnoid Cysts ?
    ### Response:
    Arachnoid cysts are cerebrospinal fluid-filled sacs that are located between the brain or spinal cord and the arachnoid membrane, one of the three membranes that cover the brain and spinal cord. Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord that arise during the early weeks of gestation.
    