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: Paired eyes were perfused in serum - free media with or without 10 ( - 7 ) M dexamethasone for 12 days .
### Output Text:
Paired eyes were perfused in serum - free media with or without 10 ( - 7 ) M <span class="chemical">dexamethasone</span> for 12 days . 