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 present experiments were performed to study the effect of the flavonoid apigenin ( 20 mg / kg intraperitoneally ( i . p . ) , 1 h before acquisition ) , on 24 h retention performance and forgetting of a step - through passive avoidance task , in young male Wistar rats .
### Output Text:
The present experiments were performed to study the effect of the <span class="chemical">flavonoid</span> <span class="chemical">apigenin</span> ( 20 mg / kg intraperitoneally ( i . p . ) , 1 h before acquisition ) , on 24 h retention performance and forgetting of a step - through passive avoidance task , in young male Wistar rats . 