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: A rat model of IUGR was established by PCE , male fetuses and adult offspring at the age of postnatal week 24 were euthanized .
### Output Text:
A rat model of IUGR was established by PCE , male fetuses and adult offspring at the age of postnatal week 24 were euthanized . 