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: These data might indicate that the generation of reactive oxygen species and activation of NF - kappaB plays a more central role in seizure - associated neuronal damage in the temporal cortex as compared to the hippocampal hilus .
### Output Text:
These data might indicate that the generation of reactive <span class="chemical">oxygen</span> species and activation of NF - kappaB plays a more central role in seizure - associated neuronal damage in the temporal cortex as compared to the hippocampal hilus . 