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 same animals the occurrence of clonic seizures was scored as to time of onset and severity for 20 min after drug administration .
### Output Text:
In the same animals the occurrence of clonic seizures was scored as to time of onset and severity for 20 min after drug administration . 