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: By day 30 , sensitivity of both splenic and bone marrow BFU - e of treated animals returned to that observed from cells of infected untreated animals .
### Output Text:
By day 30 , sensitivity of both splenic and bone marrow BFU - e of treated animals returned to that observed from cells of infected untreated animals . 