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: On the other hand , the clonidine - induced bradycardia was antagonized by prior bilateral vagotomy , but not spinal transection .
### Output Text:
On the other hand , the <span class="chemical">clonidine</span> - induced bradycardia was antagonized by prior bilateral vagotomy , but not spinal transection . 