In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Full visual electrophysiology , including flash electroretinography ( ERG ) , pattern electroretinography , multifocal ERG using the VERIS system , electro - oculography , and flash and pattern visual evoked potentials , was undertaken .
### Output Text:
Full visual electrophysiology , including flash electroretinography ( ERG ) , pattern electroretinography , multifocal ERG using the VERIS system , electro - oculography , and flash and pattern visual evoked potentials , was undertaken . 