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 histological features were improved in hippocampus of rats treated with ZnSO ( 4 ) + BCNU compared to only BCNU - treated animals .
### Output Text:
The histological features were improved in hippocampus of rats treated with <span class="chemical">ZnSO ( 4 )</span> + <span class="chemical">BCNU</span> compared to only <span class="chemical">BCNU</span> - treated animals . 