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: No statistically significant differences were observed between the two isoproterenol treated groups for severity of the induced lesions , changes in heart weight , or heart weight to body weight ratios .
### Output Text:
No statistically significant differences were observed between the two <span class="chemical">isoproterenol</span> treated groups for severity of the induced lesions , changes in heart weight , or heart weight to body weight ratios . 