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: Although 61 % of patients had a noncontributory ocular history on entry , 42 % had active uveitis on entry .
### Output Text:
Although 61 % of patients had a noncontributory ocular history on entry , 42 % had active uveitis on entry . 