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: Memory facilitation and stimulation of endogenous nerve growth factor synthesis by the acetylcholine releaser PG - 9 .
### Output Text:
Memory facilitation and stimulation of endogenous nerve growth factor synthesis by the <span class="chemical">acetylcholine</span> releaser <span class="chemical">PG - 9</span> . 