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: Surprisingly , the F1 hybrid tumors were not hemorrhagic and had hemoglobin content and outward appearance identical to that of BN .
### Output Text:
Surprisingly , the F1 hybrid tumors were not hemorrhagic and had hemoglobin content and outward appearance identical to that of BN . 