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: Nephrotoxic doses ( 40 mg / kg body wt / day ) of gentamicin were injected during the last nine days of study to the animals of groups II to IV .
### Output Text:
Nephrotoxic doses ( 40 mg / kg body wt / day ) of <span class="chemical">gentamicin</span> were injected during the last nine days of study to the animals of groups II to IV . 