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: Four groups of rats ( n = 7 ) were studied : jj and jJ rats treated either with aspirin 300 mg / kg every other day or sham - treated .
### Output Text:
Four groups of rats ( n = 7 ) were studied : jj and jJ rats treated either with <span class="chemical">aspirin</span> 300 mg / kg every other day or sham - treated . 