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 results thus suggest that intra - arterial PGE2 , given near the bladder , may initiate micturition in the normal rat chiefly by directly contracting the smooth muscle of the detrusor .
### Output Text:
The present results thus suggest that intra - arterial <span class="chemical">PGE2</span> , given near the bladder , may initiate micturition in the normal rat chiefly by directly contracting the smooth muscle of the detrusor . 