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: However , prolonged stimulation ( 8 - 12 days ) appears to represent a complex integration of both cellular hypertrophy and hyperplasia within the heart .
### Output Text:
However , prolonged stimulation ( 8 - 12 days ) appears to represent a complex integration of both cellular hypertrophy and hyperplasia within the heart . 