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: Treatment for 2 weeks with Warfarin caused massive focal calcification of the artery media in 20 - day - old rats and less extensive focal calcification in 42 - day - old rats .
### Output Text:
Treatment for 2 weeks with <span class="chemical">Warfarin</span> caused massive focal calcification of the artery media in 20 - day - old rats and less extensive focal calcification in 42 - day - old rats . 