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: The hilar ectopic granule cell population does not appear to vary systematically across the septotemporal axis , although it is associated with an increase in volume of the hilus .
### Output Text:
The hilar ectopic granule cell population does not appear to vary systematically across the septotemporal axis , although it is associated with an increase in volume of the hilus . 