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: Beginning at 8 days of age , body movement and hearing were examined for 6 and up to 17 weeks , respectively .
### Output Text:
Beginning at 8 days of age , body movement and hearing were examined for 6 and up to 17 weeks , respectively . 