within the human body. While serous membranes have a lubricative role to play in all three cavities, in the pleural cavity it has a greater role to play in the function of breathing. The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by serous membrane. Early in embryonic life visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom. Therefore, each organ becomes surrounded by serous membrane - they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall. Examples In the human body, there are three serous cavities with associated serous membranes: A serous membrane lines the pericardial cavity of the heart, and reflects back to cover the heart, much like an under-inflated balloon would form two layers surrounding a fist. Called the pericardium, this serous membrane is a two-layered sac that surrounds the entire heart except where blood vessels emerge on the heart’s superior side; The pleura is the serous membrane that surrounds the lungs in the pleural cavity; The peritoneum is the serous membrane that surrounds several organs in the abdominopelvic cavity. The tunica vaginalis is the serous membrane, which surrounds the male gonad, the testis. The two layers of serous membranes are named parietal and visceral. Between the two layers is a thin fluid filled space. The fluid is produced by the serous membranes and stays between the two layers to reduce friction between the walls of the cavities and the internal organs when they move with respect to one another, such as when the lungs inflate or the heart beats. Such movement could otherwise lead to inflammation of the organs. Development All serous membranes found in the human body formed ultimately from the mesoderm of the trilaminar embryo. The trilaminar embryo consists of three relatively flat layers of ectoderm, endoderm (also known as "entoderm") and mesoderm. As the embryo develops, the mesoderm starts to segment into three main regions: the paraxial mesoderm, the intermediate mesoderm and the lateral plate mesoderm. The lateral plate mesoderm later splits in half to form two layers bounding a cavity known as the intraembryonic coelom. Individually, each layer is known as splanchnopleure and somatopleure. The splanchnopleure is associated with the underlying endoderm with which it is