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: METHODS : We retrospectively reviewed the medical records and cranial sonograms obtained during a 1 - year period on 122 premature ( < 36 weeks of gestation ) infants .
### Output Text:
METHODS : We retrospectively reviewed the medical records and cranial sonograms obtained during a 1 - year period on 122 premature ( < 36 weeks of gestation ) infants . 