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: Rg1 , as a ginsenoside extracted from Panax ginseng , could ameliorate spatial learning impairment .
### Output Text:
<span class="chemical">Rg1</span> , as a <span class="chemical">ginsenoside</span> extracted from Panax ginseng , could ameliorate spatial learning impairment . 