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: Anencephaly has been suggested to develop from exencephaly ; however , there is little direct experimental evidence to support this , and the mechanism of transformation remains unclear .
### Output Text:
Anencephaly has been suggested to develop from exencephaly ; however , there is little direct experimental evidence to support this , and the mechanism of transformation remains unclear . 