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: Second , the anticonvulsant effect of MK - 801 in the lithium - pilocarpine model only occurred after initial periods of seizure activity .
### Output Text:
Second , the anticonvulsant effect of <span class="chemical">MK - 801</span> in the <span class="chemical">lithium</span> - <span class="chemical">pilocarpine</span> model only occurred after initial periods of seizure activity . 