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: To analyze the transformation patterns , we classified the exencephaly by size and shape of the exencephalic tissue into several types at E13 . 5 and E18 . 5 .
### Output Text:
To analyze the transformation patterns , we classified the exencephaly by size and shape of the exencephalic tissue into several types at E13 . 5 and E18 . 5 . 