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 : The cardiovascular responses to standing and head - up tilt were studied repeatedly in PD patients receiving selegiline and as the drug was withdrawn .
### Output Text:
METHODS : The cardiovascular responses to standing and head - up tilt were studied repeatedly in PD patients receiving <span class="chemical">selegiline</span> and as the drug was withdrawn . 