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 : Male Wistar rats were studied and 3 groups were formed ( 8 rats per group ) .
### Output Text:
METHODS : Male Wistar rats were studied and 3 groups were formed ( 8 rats per group ) . 