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: METHODS : Sixty dogs were either nephrectomized ( Nx , N = 38 ) or sham - operated ( Sham , N = 22 ) .
### Output Text:
METHODS : Sixty dogs were either nephrectomized ( Nx , N = 38 ) or sham - operated ( Sham , N = 22 ) . 