through this structure of relatively large units by diffusion, as in the case 

 of the oocyte nucleus which appears to be freely permeable to molecules 

 up to the size of proteins. Also in the light of what is known regarding 

 ultrastructure it is not impossible that its permeable properties are 

 subject to considerable variation. A number of electron microscopy 



Figure 4-3. Electron Micrograph of Drosophila Salivary Gland Cell 

 Showing Outpocketings (Blebs) of the Nuclear Membrane. Note association 

 of highly differentiated chromosomal material with the developing blebs. 

 Approximately 45,000x. (From Gay, H., 1956. "Chromosome-Nuclear 

 Membrane-Cytoplasmic Interrelations in Drosophila," J. Biophys. Biochem. 

 CytoL, 2, Fig. 8, Plate 137. Courtesy of Dr. Helen Gay, Carnegie Institute 

 of Washington.) 



studies have shown the nuclear envelope to be a double-layered structure 

 containing pores formed by continuities between the outer and inner 

 layers of the envelope (Figure 4-2), In certain cells, a thin membrane 

 extending across the pores and separating the contents of the nucleus 

 from those of the cytoplasm has been observed. The presence of annuli 

 or rings of electron-dense granules surrounding the pores of the envelope 

 has also been reported. These annuli appear to be associated with ex- 



76 / CHAPTER 4 



