The perinuclear space in the atrium of coldblooded vertebrates.
D000818:Animals; D005399:Fishes / Q000033:anatomy & histology*; D006654:Histology, Comparative; D066298:In Vitro Techniques; D008854:Microscopy, Electron; D009206:Myocardium / Q000166:cytology*; D012754:Sharks / Q000033:anatomy & histology*