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: In the course of developing this model , a common vehicle , propylene glycol , by itself in high doses , was found to exhibit protective properties against induced seizures and inhibited weight gain .
### Output Text:
In the course of developing this model , a common vehicle , <span class="chemical">propylene glycol</span> , by itself in high doses , was found to exhibit protective properties against induced seizures and inhibited weight gain . 