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: Ten weeks of diethylstilbestrol ( DES ) treatment caused female F344 rat pituitaries to grow to an average of 109 . 2 + / - 6 . 3 mg ( mean + / - SE ) versus 11 . 3 + / - 1 . 4 mg for untreated rats , and to become highly hemorrhagic .
### Output Text:
Ten weeks of <span class="chemical">diethylstilbestrol</span> ( <span class="chemical">DES</span> ) treatment caused female F344 rat pituitaries to grow to an average of 109 . 2 + / - 6 . 3 mg ( mean + / - SE ) versus 11 . 3 + / - 1 . 4 mg for untreated rats , and to become highly hemorrhagic . 