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 present study , effects of nicotine on catalepsy induced by morphine in mice have been investigated .
### Output Text:
In the present study , effects of <span class="chemical">nicotine</span> on catalepsy induced by <span class="chemical">morphine</span> in mice have been investigated . 