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: Dermal administration of S - 23121 at 800 mg / kg resulted in an increased incidence of embryonic death and ventricular septal defect , but retarded fetal growth was not observed as it was following oral exposure to S - 23121 .
### Output Text:
Dermal administration of <span class="chemical">S - 23121</span> at 800 mg / kg resulted in an increased incidence of embryonic death and ventricular septal defect , but retarded fetal growth was not observed as it was following oral exposure to <span class="chemical">S - 23121</span> . 