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: Animals surviving 20 , 30 , 40 , 60 min , 2 , 3 , 6 hours , 1 , 2 , and 3 days after induction of status epilepticus were perfusion - fixed , and brains processed for immunohistochemical staining of SNR .
### Output Text:
Animals surviving 20 , 30 , 40 , 60 min , 2 , 3 , 6 hours , 1 , 2 , and 3 days after induction of status epilepticus were perfusion - fixed , and brains processed for immunohistochemical staining of SNR . 