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: MEASUREMENTS AND MAIN RESULTS : Metoprolol reduced maximum heart rate 31 % , summed stress scores 29 % , and summed difference scores 43 % versus control .
### Output Text:
MEASUREMENTS AND MAIN RESULTS : <span class="chemical">Metoprolol</span> reduced maximum heart rate 31 % , summed stress scores 29 % , and summed difference scores 43 % versus control . 