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: For both experiments the drug intake led to significant increases in PRL secretion , acting preferentially on tonic secretion as pulse amplitude and frequency did not differ significantly from corresponding control values .
### Output Text:
For both experiments the drug intake led to significant increases in PRL secretion , acting preferentially on tonic secretion as pulse amplitude and frequency did not differ significantly from corresponding control values . 