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: The continuous subcutaneous administration of mesna has the advantage of not requiring intravenous access .
### Output Text:
The continuous subcutaneous administration of <span class="chemical">mesna</span> has the advantage of not requiring intravenous access . 