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: On the 13th day of each period , heart rate was recorded continuously during a typical , ambulant 24 - hour period .
### Output Text:
On the 13th day of each period , heart rate was recorded continuously during a typical , ambulant 24 - hour period . 