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 absence of caffeine , acetaminophen ( up to 300 mg / kg ) did not modify the seizures induced by maximal electroshock and did not alter the convulsant dose of pentylenetetrezol in mice ( tests performed by the Anticonvulsant Screening Project of NINCDS ) .
### Output Text:
In the absence of <span class="chemical">caffeine</span> , <span class="chemical">acetaminophen</span> ( up to 300 mg / kg ) did not modify the seizures induced by maximal electroshock and did not alter the convulsant dose of <span class="chemical">pentylenetetrezol</span> in mice ( tests performed by the Anticonvulsant Screening Project of NINCDS ) . 