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: Although several new anticoagulant drugs are in development , heparin remains the drug of choice for most anticoagulation needs .
### Output Text:
Although several new anticoagulant drugs are in development , <span class="chemical">heparin</span> remains the drug of choice for most anticoagulation needs . 