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: Aged GF Sprague - Dawley ( SD ) rats have not developed prostate cancer spontaneously .
### Output Text:
Aged GF Sprague - Dawley ( SD ) rats have not developed prostate cancer spontaneously . 