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: The present study was undertaken to examine the role of sleep disturbance , induced by clomipramine administration , on the secretory rate of prolactin ( PRL ) in addition to the direct drug effect .
### Output Text:
The present study was undertaken to examine the role of sleep disturbance , induced by <span class="chemical">clomipramine</span> administration , on the secretory rate of prolactin ( PRL ) in addition to the direct drug effect . 