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: The degree and time course of development of tolerance to daily administration of sub - effective dose combinations of ketamine and morphine were similar .
### Output Text:
The degree and time course of development of tolerance to daily administration of sub - effective dose combinations of <span class="chemical">ketamine</span> and <span class="chemical">morphine</span> were similar . 