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: Differential effects of 17alpha - ethinylestradiol on the neutral and acidic pathways of bile salt synthesis in the rat .
### Output Text:
Differential effects of <span class="chemical">17alpha - ethinylestradiol</span> on the neutral and acidic pathways of <span class="chemical">bile salt</span> synthesis in the rat . 