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: Mast cells appeared in the connective and muscular layers of the bladder at a much higher number in DBA / 2 mice than in C57BL / 6 mice or untreated controls .
### Output Text:
Mast cells appeared in the connective and muscular layers of the bladder at a much higher number in DBA / 2 mice than in C57BL / 6 mice or untreated controls . 