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: The area of histochemical injury ( as a percent of the cross - sectional area of the hemisphere ) was less in the hypertensive group ( 33 + / - 3 % vs 21 + / - 2 % , P less than 0 . 05 ) .
### Output Text:
The area of histochemical injury ( as a percent of the cross - sectional area of the hemisphere ) was less in the hypertensive group ( 33 + / - 3 % vs 21 + / - 2 % , P less than 0 . 05 ) . 