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: Specific gravity ( SG ) was determined in the subcortex and in two sites in the cortex ( core and periphery of the ischemic territory ) .
### Output Text:
Specific gravity ( SG ) was determined in the subcortex and in two sites in the cortex ( core and periphery of the ischemic territory ) . 