Diversity at the Subcellular Level and Its Significance 145 



The outer membrane is studded with ribosomes, like rough cisternae 

 of the ER, and the inner one is in intimate association with the periph- 

 eral chromatin of the interphase nucleus. The latter intimacy is such 

 as to suggest an active and important physiologic exchange, or what 

 have you between the envelope membrane and cavities and the 

 genetic material of the nucleus. Perforations or fenestrae in the 

 envelope connect, apparently without structural interception, the 

 matrix of the cytoplasm and less dense nucleoplasm of the nucleus 

 and should provide for a free exchange of fairly large molecules. 



In some cells the endoplasmic reticulum is represented almost 

 solely by the nuclear envelope, other parts having disappeared in the 

 later stages of cytoplasmic differentiation (Fig. 10). In another and 

 more commonly encountered structural expression or pattern, there 

 are many lamellar sacs or cisternae in parallel array and this form 

 corresponds to the strongly basophilic component of the cytoplasm 

 long known as ergastoplasm. In other instances the system is repre- 

 sented by a three-dimensional lattice of particle-free tubules (Fig. 6). 

 The diversity of forms ranges widely between the extremes shown, 

 for example, by the cells of the vertebrate pancreas and the interstitial 

 cell of the vertebrate testis. The interesting thing is that the structural 

 pattern of the ER is constant (within limits) for any cell type and it 

 is possible for the experienced observer to identify a cell on the basis 

 of ER morphology alone. These patterns and their functional relation- 

 ships are discussed more fully below. 



From these and other observations on the morphology and varia- 

 tions in this system, it becomes evident that the ER is a complex, finely 

 divided vacuolar system which ramifies and extends to all parts of the 

 cytosome. It effectively creates in the cytoplasm a structureless and 

 sometimes discontinuous internal phase separated by a membrane 

 from the continuous phase of the cytoplasmic matrix. It has been sug- 

 gested, on the basis of these and related observations, that the ER 

 provides for the channeled diffusion and segregation of metabolites. 

 It is probable that the membrane is electrically polarized. And cyto- 

 chemical studies support the oft-expressed suggestion that the large 

 surfaces of the system provide for the support and patterned disposi- 

 tion of enzymes within the cytoplasm. 



Direr. situ in lilt patterns 



One of the most striking features of this complex membrane-limited 

 structure is its tendency to adopt similar patterns in cells of the same 

 type. These patterns, it would seem, are determined by the macro- 



