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: Microscopic observation showed the configuration of exencephaly at E13 . 5 , frequent hemorrhaging and detachment of the neural plate from surface ectoderm in the exencephalic head at E15 . 5 , and multiple modes of reduction in the exencephalic tissue at E18 . 5 .
### Output Text:
Microscopic observation showed the configuration of exencephaly at E13 . 5 , frequent hemorrhaging and detachment of the neural plate from surface ectoderm in the exencephalic head at E15 . 5 , and multiple modes of reduction in the exencephalic tissue at E18 . 5 . 