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 : Seventy Swiss albino female mice ( 25 - 35 g ) were divided into 4 groups : amphetamine - free ( control ) , amphetamine , 50 , and 100 mg / kg DHEA .
### Output Text:
METHODS : Seventy Swiss albino female mice ( 25 - 35 g ) were divided into 4 groups : <span class="chemical">amphetamine</span> - free ( control ) , <span class="chemical">amphetamine</span> , 50 , and 100 mg / kg <span class="chemical">DHEA</span> . 