Transmission electron micrographs of non-actinomycin D-treated 4-cell porcine embryos. a) Nucleus with an NPB and a fibrillogranular nucleolus (N). ×4130. b) Detail showing the fibrillogranular nucleolus established on the surface of an NPB. Note the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). ×17 800