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: None of the cows treated with vitamin D3 showed signs of milk fever during the peripartal period ; however , 22 % of the control cows developed clinical signs of milk fever during this period .
### Output Text:
None of the cows treated with <span class="chemical">vitamin D3</span> showed signs of milk fever during the peripartal period ; however , 22 % of the control cows developed clinical signs of milk fever during this period . 