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: OBJECTIVE : To describe the incidence of haematological , renal , hepatic and gastrointestinal toxicities in tumour - bearing dogs receiving 1 - ( 2 - chloroethyl ) - 3 - cyclohexyl - 1 - nitrosourea ( CCNU ) .
### Output Text:
OBJECTIVE : To describe the incidence of haematological , renal , hepatic and gastrointestinal toxicities in tumour - bearing dogs receiving <span class="chemical">1 - ( 2 - chloroethyl ) - 3 - cyclohexyl - 1 - nitrosourea</span> ( <span class="chemical">CCNU</span> ) . 