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: After a 6 - week treatment period and additional observation for 2 weeks , the mice were sacrificed .
### Output Text:
After a 6 - week treatment period and additional observation for 2 weeks , the mice were sacrificed . 