18 Philip Siekevitz 



of nearly all cells (Palade and Porter, 1954; Palade, 1956a 

 and b; Porter, 1955, 1957; Porter and Palade, 1957). Since 

 this system is bounded by a membrane we can speak of an 

 intraluminal space and of a periluminal (cytoplasmic matrix) 

 space. This structure is not uniform throughout a particular 

 cell but it can be visually differentiated into local variations. 

 In some parts of the cell the membranes of this arrangement 

 are naked, while in other parts they are covered by ribo- 

 nucleoprotein particles (about 150 a in diameter). In various 

 regions of the cell the canals of the system are seemingly 

 rigidly disposed in relation to each other, while in other parts 

 there is a helter-skelter, disordered ordination of the vesicles. 

 In the pancreatic acinar cell (Fig. 1) the orderly arrangement 

 takes the form of a system of parallel canals (cisternae) with 

 particles on their membranes and with a uniformly repeating 

 distance between them, while even in the same cell there is a 

 region, the Golgi zone, where the membranes are bare and are 

 disposed in stacks and swarms of vesicles (Fig. 2). In the 

 muscle cell we find a distinctive, lace-like network surrounding 

 the fibrils (Porter and Palade, 1957). The visualization of the 

 ER in many other cell types can be attained by glancing 

 through the references mentioned above. On the basis of a 

 unitary hypothesis, there is very little reason to doubt that 

 the local variations within a single cell, or the variations in 

 character between different cells, are just variations, and that 

 all are derived from a common system of membrane-limited 

 lumina (Palade, 1956a; Porter, 1957). This system fragments 

 upon homogenization and gives rise to the microsome fraction 

 in the usual differential centrifugation technique (Palade and 

 Siekevitz, 1956a). 



Now I propose to take you on a brief journey from the outer 

 spaces of the extracellular fluid to the inner ones of the 

 nucleus. Several workers have demonstrated that the cell 

 membrane in most cells is at some places invaginated to form 

 inpocketings into the cell (Palade, 1956a; Parks and Chi- 

 quoine, 1956; Palay, 1958). Figures 3, 4 and 5 show these 

 invaginations as they occur in a spleen macrophage, in a 



