20 Philip Siekevitz 



by the presence of the typical particles on its surface and by 

 its continuity with other ER membranes (Fig. 6). Thus, what 

 at first glance was thought to be a double nuclear membrane 

 turns out to be an extension of the membrane of the ER. 

 The space between these two membranes, the perinuclear 

 space, is continuous with the intraluminal space between the 

 membranes of the ER and is, in fact, just an extension of it. 

 Thus we can say that, unlike the mitochondria, the nucleus 

 is closely surrounded by the cavities of the ER, with only one 

 membrane separating the contents of the nucleus and the 

 contents of the intracisternal space. Watson (1954, 1955) has 

 also shown the existence of pores, which are actually windows 

 through the perinuclear space, so that the contents of the 

 nucleus are continuous with the contents of the cytoplasmic 

 matrix (cf. Fig. 7). 



What are the physiological implications of this brief 

 representation of the cell? It has been accepted for a long 

 time that the passage of molecules into and out of the cell 

 occurs solely by a discrete individual passage of compounds 

 through a fixed cell membrane, either by a carrier mechanism 

 or by passage through pores or by "solubilization" in a 

 lipoidal phase. Nowadays, we are beginning to understand 

 that in all cells, with the probable exception of the mature 

 mammalian red blood cell, another process also occurs, and 

 probably simultaneously with the first. This second process is 

 emphasized in Fig. 7, which shows a stylized and idealized 

 representation of that most common of cells, the typical cell. 

 The extracellular fluid is shown to be brought into the cell, in 

 part, by means of actual movements of the membranes. 

 Whether the vesicles so formed are broken down and the 

 contents released into the general cytoplasmic matrix — and 

 this is the accepted view — or whether these vesicles finally 

 communicate with the vesicular spaces of the ER, or whether 

 both processes take place, is not known. Nevertheless, it is 

 quite clear that by this mechanism the membranous surface 

 of the cell is multiplied many times and what is probably 

 more important, the extracellular fluids are brought into the 



