Figure 4-6. Electron Micrograph of Portion of Late Prophase; Nucleus in 

 a Cell from the Outer Cortex of Onion Root Tip. Three sectors of the nuclear 

 envelope are visible, one of which still retains a connection with a membrane 

 element of the endoplasmic reticulum. Outside the remnants of the nuclear 

 envelope and encircling the nucleus is a zone from which mitochondria and 

 plastids are excluded. Large vesicles with a limiting membrane and homo- 

 geneous content, called phragmosomes, begin to appear with increasing fre- 

 quency at this stage of cell division. (From Porter, K. R. and Machado, R. D., 

 1960. "Studies on the Endoplasmic Reticulum. IV. Its Form and Distribution 

 During Mitosis in Cells of Onion Root Tip," /. Biophys. Biochem. CytoL, 7, 

 Fig. 5, Plate 84. Courtesy of Dr. Keith R. Porter, The Rockefeller Institute.) 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 79 



