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: Ophthalmological examinations in over 1100 patients treated with one or the other agent have revealed no evidence of significant short term ( up to 2 years ) cataractogenic potential .
### Output Text:
Ophthalmological examinations in over 1100 patients treated with one or the other agent have revealed no evidence of significant short term ( up to 2 years ) cataractogenic potential . 