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: Serotonin was depleted beginning on postnatal day 26 with parachlorophenylalanine ( PCPA 100 mg / kg , every other day ) ; controls received saline .
### Output Text:
<span class="chemical">Serotonin</span> was depleted beginning on postnatal day 26 with <span class="chemical">parachlorophenylalanine</span> ( <span class="chemical">PCPA</span> 100 mg / kg , every other day ) ; controls received saline . 