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: It was found that the transformation of exencephalic tissue was not simply size - dependent , and all cases of anencephaly at E18 . 5 resulted from embryos with a large amount of exencephalic tissue at E13 . 5 .
### Output Text:
It was found that the transformation of exencephalic tissue was not simply size - dependent , and all cases of anencephaly at E18 . 5 resulted from embryos with a large amount of exencephalic tissue at E13 . 5 . 