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: Delayed toxicity of cyclophosphamide on the bladder of DBA / 2 and C57BL / 6 female mouse .
### Output Text:
Delayed toxicity of <span class="chemical">cyclophosphamide</span> on the bladder of DBA / 2 and C57BL / 6 female mouse . 