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: Staining for serum - proteins occurred in a patchy manner throughout the forebrain during the first hours .
### Output Text:
Staining for serum - proteins occurred in a patchy manner throughout the forebrain during the first hours . 