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: We observed the exencephaly induced by 5 - azacytidine at embryonic day 13 . 5 ( E13 . 5 ) , let the embryos develop exo utero until E18 . 5 , and re - observed the same embryos at E18 . 5 .
### Output Text:
We observed the exencephaly induced by <span class="chemical">5 - azacytidine</span> at embryonic day 13 . 5 ( E13 . 5 ) , let the embryos develop exo utero until E18 . 5 , and re - observed the same embryos at E18 . 5 . 