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 contrast , the highest dose of melatonin ( 50 mg / kg ) caused a paradoxical increase in the latency and produced a sustained reduction of the duration of narcosis , and a reduction in mortality rate .
### Output Text:
In contrast , the highest dose of <span class="chemical">melatonin</span> ( 50 mg / kg ) caused a paradoxical increase in the latency and produced a sustained reduction of the duration of narcosis , and a reduction in mortality rate . 