How can I store the output of a deep learning layer to a variable ?

Inorder to implement an academic project, I need to use the output of specific layers in a deepnet. The main part of my code goes like this:

    dbPath = <path to my image database>;
    imds = imageDatastore(dbPath,'IncludeSubfolders', true,'LabelSource','foldernames');
    [trainImgs,testImgs] = splitEachLabel(imds,0.6,'randomized');
    I = imread(<my image>)
    cnnNet = vgg16;
    label = classify(cnnNet, I)

Now , I need to work on the outputs of the layer 'fc8' (the final Fully Connected Layer). But i couldn't find how to store the output of a specific layer to a variable. Please help