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: Structure - activity and dose - effect relationships of the antagonism of picrotoxin - induced seizures by cholecystokinin , fragments and analogues of cholecystokinin in mice .
### Output Text:
Structure - activity and dose - effect relationships of the antagonism of <span class="chemical">picrotoxin</span> - induced seizures by <span class="chemical">cholecystokinin</span> , fragments and analogues of <span class="chemical">cholecystokinin</span> in mice . 