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: Extended follow - up studies of up to 8 years have demonstrated long - term persistence of apomorphine efficacy .
### Output Text:
Extended follow - up studies of up to 8 years have demonstrated long - term persistence of <span class="chemical">apomorphine</span> efficacy . 