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: This study was designed to evaluate the ameliorative effects of metformin on seizures , cognitive impairment and brain oxidative stress markers observed in pentylenetetrazole - induced kindling animals .
### Output Text:
This study was designed to evaluate the ameliorative effects of <span class="chemical">metformin</span> on seizures , cognitive impairment and brain oxidative stress markers observed in <span class="chemical">pentylenetetrazole</span> - induced kindling animals . 