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: METHODS : The drugs were given orally , in doses of 50 mg / kg for five consecutive days and the experiments were performed on the fifth day .
### Output Text:
METHODS : The drugs were given orally , in doses of 50 mg / kg for five consecutive days and the experiments were performed on the fifth day . 