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: To quantify the effects of gentamicin , kanamycin and netilmicin on renal protein reabsorption and accumulation , these drugs were administered to rats intraperitoneally ( 30 mg / kg / day ) for 7 , 14 or 21 days .
### Output Text:
To quantify the effects of <span class="chemical">gentamicin</span> , <span class="chemical">kanamycin</span> and <span class="chemical">netilmicin</span> on renal protein reabsorption and accumulation , these drugs were administered to rats intraperitoneally ( 30 mg / kg / day ) for 7 , 14 or 21 days . 