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: Hemorrhagic cystitis is a major potential toxicity of ifosfamide that can be prevented by administering mesna along with the cytotoxic agent .
### Output Text:
Hemorrhagic cystitis is a major potential toxicity of <span class="chemical">ifosfamide</span> that can be prevented by administering <span class="chemical">mesna</span> along with the cytotoxic agent . 