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: Electron microscopic investigations of the cyclophosphamide - induced lesions of the urinary bladder of the rat and their prevention by mesna .
### Output Text:
Electron microscopic investigations of the <span class="chemical">cyclophosphamide</span> - induced lesions of the urinary bladder of the rat and their prevention by <span class="chemical">mesna</span> . 