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: Fetal serum concentrations at onset of seizures in the mother were similar to maternal brain and CSF concentrations and correlated significantly with the former .
### Output Text:
Fetal serum concentrations at onset of seizures in the mother were similar to maternal brain and CSF concentrations and correlated significantly with the former . 