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: The total content of hydroxyproline remained stable during the first 2 days of treatment but increased 46 % after 4 days of therapy .
### Output Text:
The total content of <span class="chemical">hydroxyproline</span> remained stable during the first 2 days of treatment but increased 46 % after 4 days of therapy . 