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: Each concentration of progesterone ( 6 . 25 , 12 . 5 , 25 , and 50 micrograms / ml ) caused a significant and concentration - dependent reduction in the AD50 for bupivacaine .
### Output Text:
Each concentration of <span class="chemical">progesterone</span> ( 6 . 25 , 12 . 5 , 25 , and 50 micrograms / ml ) caused a significant and concentration - dependent reduction in the AD50 for <span class="chemical">bupivacaine</span> . 