The layer in the middle is made up of various proteins.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/34745138@N00">kaibara87</a> from Italy
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/34745138@N00/2437264760/">Proteins Layer</a>