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: Cortical dysplasia is a malformation characterized by defects in proliferation , migration and maturation .
### Output Text:
Cortical dysplasia is a malformation characterized by defects in proliferation , migration and maturation . 