THE STRUCTURE OF THE CELL 11 



exitending through most parts of the cytoplasm. It commonly 

 consists of rounded components (fig. 2, A, r), joined by narrower 

 elements that may be called trabeculae (t). The material forming 

 the network is seen in sectional view to be dark externally and 

 pale internally, and this has given rise to the opinion that the 

 reticulum is hollow. If so, the rounded components are vesicles 

 and the trabeculae are tubes, and all or nearly all the cavities are 

 continuous. There is, however, no rigorous proof that this is so. 

 It may be simply that the external layer has a greater affinity for 

 osmium than the internal part: we do not know which is the 

 denser. 



The swollen components of the endoplasmic reticulum are not 

 always rounded. Sometimes they are flattened and seem to 

 resemble sacks piled one on another. In section the flattened 

 *sacks' appear as elongated spaces (fig. 2, B, er), though here 

 again we have no proof that the objects are in fact hollow. They 

 are connected with one another, like the rounded elements, by 

 trabeculae, which again may or may not be tubular. 



Although the flattened components of the endoplasmic reticu- 

 lum are not individually recognizable under the light-micro- 

 scope, yet large piles of them do sometimes reach microscopical 

 dimensions. A well-known example is the striated material seen 

 at the base of the exocrine cells of the pancreas. Striated cyto- 

 plasm of this kind was long ago named ergastoplasme,^^^ ®^ and 

 the word is still used to denote this particular modification of the 

 endoplasmic reticulum. The Nissl bodies of the neurones of 

 vertebrates are similar in submicroscopic structure to the basal 

 cytoplasm of the exocrine cell.^*^ 



There is not yet any sure knowledge of the significance for the 

 life of the cell of the object that is represented by endoplasmic 

 reticulum in electron-micrographs. 



The name 'endoplasmic reticulum' is an unfortunate one. The 

 object was first recognized in certain sarcoma cells, in which a 

 very narrow, hyaline, marginal zone could be distinguished from 

 the internal endoplasm. The reticulum was called endoplasmic 

 because it was retricted to the inner zone.^^^' ^^^ The difi"erentia- 

 tion of a special external zone or ectoplasm from a less hyaline 



