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: Onset of hyperammonemic encephalopathy varied from 0 . 5 to 5 days ( mean : 2 . 6 + / - 1 . 3 days ) after the initiation of chemotherapy .
### Output Text:
Onset of hyperammonemic encephalopathy varied from 0 . 5 to 5 days ( mean : 2 . 6 + / - 1 . 3 days ) after the initiation of chemotherapy . 