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: Animals were killed between 30 and 60 days later , and brain sections were processed for GAP43 immunohistochemistry .
### Output Text:
Animals were killed between 30 and 60 days later , and brain sections were processed for GAP43 immunohistochemistry . 