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: At the end of the experimental period , 60 % of the animals treated with apigenin still did not step through .
### Output Text:
At the end of the experimental period , 60 % of the animals treated with <span class="chemical">apigenin</span> still did not step through . 