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: One hour after the administration of gamma - HCH , the activity of seizure - inducing agents was increased , regardless of their mechanism , while 24 h after gamma - HCH a differential response was observed .
### Output Text:
One hour after the administration of <span class="chemical">gamma - HCH</span> , the activity of seizure - inducing agents was increased , regardless of their mechanism , while 24 h after <span class="chemical">gamma - HCH</span> a differential response was observed . 