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: A developmental analysis of clonidine ' s effects on cardiac rate and ultrasound production in infant rats .
### Output Text:
A developmental analysis of <span class="chemical">clonidine</span> ' s effects on cardiac rate and ultrasound production in infant rats . 