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: 17beta - Estradiol did not alter the onset of first clonus in ovariectomized rats but accelerated it in males .
### Output Text:
<span class="chemical">17beta - Estradiol</span> did not alter the onset of first clonus in ovariectomized rats but accelerated it in males . 