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: The flavonoid apigenin delays forgetting of passive avoidance conditioning in rats .
### Output Text:
The <span class="chemical">flavonoid</span> <span class="chemical">apigenin</span> delays forgetting of passive avoidance conditioning in rats . 