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: Initial testing in a time - dependent forgetting task employing a 24 - h delay between training and testing showed that metrifonate improved object recognition ( at 10 and 30 mg / kg , p . o . ) , whereas Ro4368554 was inactive .
### Output Text:
Initial testing in a time - dependent forgetting task employing a 24 - h delay between training and testing showed that <span class="chemical">metrifonate</span> improved object recognition ( at 10 and 30 mg / kg , p . o . ) , whereas <span class="chemical">Ro4368554</span> was inactive . 