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: Sequential observations of exencephaly and subsequent morphological changes by mouse exo utero development system : analysis of the mechanism of transformation from exencephaly to anencephaly .
### Output Text:
Sequential observations of exencephaly and subsequent morphological changes by mouse exo utero development system : analysis of the mechanism of transformation from exencephaly to anencephaly . 