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 cytogenetic action of ifosfamide , mesna , and their combination on peripheral rabbit lymphocytes : an in vivo / in vitro cytogenetic study .
### Output Text:
The cytogenetic action of <span class="chemical">ifosfamide</span> , <span class="chemical">mesna</span> , and their combination on peripheral rabbit lymphocytes : an in vivo / in vitro cytogenetic study . 