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 animals treated with threshold doses of picrotoxin , the average number of seizures was 2 . 3 + / - 1 . 2 , and average seizure duration was 39 . 5 + / - 8 . 4s .
### Output Text:
In the animals treated with threshold doses of <span class="chemical">picrotoxin</span> , the average number of seizures was 2 . 3 + / - 1 . 2 , and average seizure duration was 39 . 5 + / - 8 . 4s . 