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: Apoptosis was assessed by fluorescence microscopy in Hoechst 33342 - and propidium iodide stained cell samples .
### Output Text:
Apoptosis was assessed by fluorescence microscopy in <span class="chemical">Hoechst 33342</span> - and <span class="chemical">propidium iodide</span> stained cell samples . 