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: While powerful antiparkinsonian effects had been observed as early as 1951 , the potential of treating fluctuating Parkinson ' s disease by subcutaneous administration of apomorphine has only recently become the subject of systematic study .
### Output Text:
While powerful antiparkinsonian effects had been observed as early as 1951 , the potential of treating fluctuating Parkinson ' s disease by subcutaneous administration of <span class="chemical">apomorphine</span> has only recently become the subject of systematic study . 