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: Metformin protects against seizures , learning and memory impairments and oxidative damage induced by pentylenetetrazole - induced kindling in mice .
### Output Text:
<span class="chemical">Metformin</span> protects against seizures , learning and memory impairments and oxidative damage induced by <span class="chemical">pentylenetetrazole</span> - induced kindling in mice . 