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: Double staining revealed numerous interstitial apoptotic cells to stain for ED1 , a marker for monocytes / macrophages .
### Output Text:
Double staining revealed numerous interstitial apoptotic cells to stain for ED1 , a marker for monocytes / macrophages . 