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: STUDY OBJECTIVE : The aim of the study was to compare the effect of pretreatment with remifentanil 1 microg / kg and the effect of gender on the incidence of myoclonus after anesthesia induction with etomidate .
### Output Text:
STUDY OBJECTIVE : The aim of the study was to compare the effect of pretreatment with <span class="chemical">remifentanil</span> 1 microg / kg and the effect of gender on the incidence of myoclonus after anesthesia induction with <span class="chemical">etomidate</span> . 