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: Variations and malformations were similar when ASA was administered as a single dose or during the period of organogenesis ( GDs 6 to 17 ) .
### Output Text:
Variations and malformations were similar when <span class="chemical">ASA</span> was administered as a single dose or during the period of organogenesis ( GDs 6 to 17 ) . 