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: Whereas certain in vivo data suggest that these effects are alpha - mediated , other in vitro data suggest the opposite .
### Output Text:
Whereas certain in vivo data suggest that these effects are alpha - mediated , other in vitro data suggest the opposite . 