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: METHODS : Drugs were separately , orally once daily dosed to pregnant rats from day 8 to 21 ( GD1 = plug day ) .
### Output Text:
METHODS : Drugs were separately , orally once daily dosed to pregnant rats from day 8 to 21 ( GD1 = plug day ) . 