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 extent of silver - stained CA3 and CA1 hippocampal neurons was evaluated 2 days after SE .
### Output Text:
The extent of <span class="chemical">silver</span> - stained CA3 and CA1 hippocampal neurons was evaluated 2 days after SE . 