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: After 2 weeks , the rats were subjected to ganglionic blockade and 2 days later , an infusion of clonidine .
### Output Text:
After 2 weeks , the rats were subjected to ganglionic blockade and 2 days later , an infusion of <span class="chemical">clonidine</span> . 