Endoplasmic reticulum was discovered by Porter et al $(1945)$. It is a network of membranous sac like structure such as cisternae, vesicles and narrow tubules. On the basis of presence or absence of ribosomes on its surface these structure are of two types.
Rough Endoplasimc Reticulm $(RER) $ with ribosomes on its surface is prominently present in protein synthesizing cells. It is also called as ergastoplasm. Smooth Endoplasmic Reticulms $(SER)$ lack ribosomes on its surface. It is also known as sarcoplasm. These structure predominantly occur in fat synthesising cells.