Units of Replication 25 



plasmodesmata, there is a pair of plasma membranes. It is as if the 

 tissues of Metazoa were composed of cells stuck together, whereas 

 those of plants result from the more or less incomplete partition of 

 "protoplasm." The boundary between animal cells may be exceed- 

 ingly complex. Structural and chemical properties of the intercellular 

 region suggest that specialization of the periphery of the cell may 

 play an important role in cellular differentiation. 



The most conspicuous structure within most cells is, of course, the 

 nucleus. Recent electron micrographs show that the nucleus is 

 bounded by a pair of membranes; the term nuclear envelope may be 

 used to refer to both. The inner membrane appears to surround the 

 nuclear contents like a sack. The outer membrane, however, is clearly 

 continuous with a membrane system that permeates the cytoplasm 

 to a greater or lesser extent. Thus the cell appears to be penetrated 

 by a system of tubes, canals, vesicles, and cisternae (the amount 

 and type depending upon the nature of the cell and its state of ac- 

 tivity) called the endoplasmic reticulum. In a sense, the nuclear 

 contents are outside the cell, for the cytoplasmic membranes are 

 continuous also with the plasma membrane. The inner and outer 

 membranes of the nuclear envelope are connected, however, since 

 they are both perforated by pores distributed rather regularly over 

 the surface of the envelope. These pores lead to the cytoplasm sur- 

 rounding the endoplasmic reticulum. 



Electron micrographs of the cytoplasm around the endoplasmic 

 reticulum show it to be far from homogeneous. The tubes of which 

 the reticulum is composed are associated with small dense granules 

 in synthetically active cells. These are absent where the tubes are 

 continuous with the plasma membrane. Occasionally these granules, 

 which are rich in ribonucleo-protein, are found in the intervening 

 cytoplasm. They are known as ribosomes and, as mentioned in Chap. 

 1, are thought to be concerned with protein synthesis. Particles 

 known as microsomes which have been studied by physiologists 

 appear to be artifacts: aggregations of ribosomes and endoplasmic 

 reticulum that appear when the cell is fractionated. Ribosomes do 

 not occur where the endoplasmic reticulum is continuous, with more 

 or less flattened, concentrically arranged cisternae making up the 

 Golgi material of both plant and animal cells. The Golgi complex 

 is difficult to isolate from the other cell organelles, and its precise 

 function is not yet known. 



Scattered among the tubes and vesicles comprising the endo- 

 plasmic reticulum are found the mitochondria of plant and animal 

 cells. These spherical or tube-shaped structures also have a double- 

 membrane boundary, the inner membrane being thrown into a series 



