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: Twenty patients received 27 courses of ifosfamide administered as a 24 - hour continuous infusion for 14 days without Mesna .
### Output Text:
Twenty patients received 27 courses of <span class="chemical">ifosfamide</span> administered as a 24 - hour continuous infusion for 14 days without <span class="chemical">Mesna</span> . 