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: Signs of vitamin D3 toxicity were not observed in nonlactating nonpregnant cows ; however , pregnant cows commonly developed severe signs of vitamin D3 toxicity and 10 of 17 cows died .
### Output Text:
Signs of <span class="chemical">vitamin D3</span> toxicity were not observed in nonlactating nonpregnant cows ; however , pregnant cows commonly developed severe signs of <span class="chemical">vitamin D3</span> toxicity and 10 of 17 cows died . 