1 © Springer Nature Switzerland AG 2021 O. Engin (ed.), Colon Polyps and Colorectal Cancer, https://doi.org/10.1007/978-3-030-57273-0_1 Anatomy of the Colon, Rectum, and Anus Semra Salimoglu, Gizem Kilinc, and Bulent Calik Embryology of the Colon and Anorectic Area A complete understanding of the colorectal anatomy is closely related to under- standing its embryological development. The primitive gut develops from the yolk sac’s endoderm. At the beginning of the third week of the fetus, the gut tube divides into three sections. Its sections are named from cranial to caudal as foregut, midgut, and hindgut. The development of the primary intestinal loop occurs with a rapid elongation of the midgut and its mesentery [1]. As a result of this rapid growth in the intestinal length and the simultaneous growth of the liver, the abdomen cannot accommodate all the intestines within itself for a temporary period of time. Thus, the intestinal loops go into the extraembryonic coelomic cavity within the umbilical cord in the sixth week. That is called as physi- ological umbilical herniation. During the tenth week, herniated intestinal loops begin to return to the abdominal cavity. Although the factors initiating this return are not known clearly, the regression of the mesonephyric kidney, the decline in the growth rate of the liver, and the actual enlargement of the abdominal cavity are considered to play a signifcant role. The distal 1/3 of the transverse colon, the descending colon, the sigmoid colon, the rectum, and the upper parts of the anal canal develop from the hindgut [2]. The endoderm of the hindgut is also the origin of the bladder and the mucosa of the urethra (Fig. 1.1a). In the later phases of the development, a transverse ridge which is called the urorectal septum arises in the angle between the allantois and the hindgut (Fig. 1.1b). This septum grows downward separating the cloaca into two sections as the primitive urogenital sinus in the anterior and the anorectal canal in S. Salimoglu (*) · G. Kilinc · B. Calik Department of General Surgery, Tepecik Training and Research Hospital, Health Sciences University, Izmir, Turkey 1