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 ultrastructural studies in the initial stage revealed significant lack of particles of perfused ruthenium red on the lamina rara externa and marked changes in epithelial cell cytoplasm .
### Output Text:
The ultrastructural studies in the initial stage revealed significant lack of particles of perfused <span class="chemical">ruthenium</span> red on the lamina rara externa and marked changes in epithelial cell cytoplasm . 