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: Subjects were randomly assigned to 4 - hourly or 8 - hourly regimens within CDC groups while taking 600 and 1200 mg / day .
### Output Text:
Subjects were randomly assigned to 4 - hourly or 8 - hourly regimens within CDC groups while taking 600 and 1200 mg / day . 