CH2 - Glossary of Key Terms and Concepts for Chapter 2...

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Glossary of Key Terms and Concepts for Chapter 2 Allantois - This ventral diverticulum of the hindgut becomes a superior extension of the primitive urogenital sinus (and eventually the bladder) after partitioning of the cloaca (see Chapters 9 and 10). It is rudimentary in humans and is obliterated during development to form the urachus or median (middle) umbilical ligament. Rarely, in humans, it may give rise to urachal cysts or fistulae resulting in the exudation of urine at the umbilicus. In birds and reptiles, it is enlarged and flattened just within the shell membrane to serve as an organ of respiration and nutrition. Amnioblasts - These cells differentiate from the epiblast at the end of the first week to form the amnioblast layer of the amniotic membrane. Even before the extraembryonic somatopleuric mesoderm becomes associated with the amnioblasts, the space between the amnioblasts and the epiblast begins to fill with fluid to form the amniotic sac. Amnion - This term refers to the membrane around the amniotic cavity and is synonymous with "amniotic sac". It is composed of two cell types, an inner layer of amnioblasts, derived from the epiblast, and an outer layer of extraembryonic somatopleuric mesoderm. The amniotic cavity is filled with amniotic fluid, which provides protection and space for growth of the embryo and fetus. Bilaminar germ disc - After the epiblast and hypoblast differentiate from each other in the embryoblast, the embryo is called a bilaminar (two-layered) germ disc. Chorion - This term refers to the outermost fetal membrane that surrounds the chorionic cavity. It is synonymous with "chorionic sac" and "chorionic plate" and consists of three kinds of tissues: syncytiotrophoblast, cytotrophoblast, and extraembryonic somatopleuric mesoderm and their derivatives. Chorionic cavity - The chorionic cavity appears during the second week of development as vacuoles within the extraembryonic reticulum (between the yolk sac and the cytotrophoblast) coalesce into a single cavity. As this is occurring, migrating cells of extraembryonic mesoderm become associated with the inner surface of the cytotrophoblast and the outer surface of the yolk sac. Thus, the lining of the chorionic cavity consists of extraembryonic mesoderm. By the end of the 8th week, the growth of the amniotic cavity almost completely obliterates the chorionic cavity as chorionic fluid is resorbed and the extraembryonic somatopleuric layers of the amnion and chorion loosely fuse together (see extraembryonic coelomic cavity below). Coagulation plug - This fibrin coagulum forms at the site of entry of the invading blastocyst in the endometrial lining of the uterus. Connecting stalk - The connecting stalk at first is a broad mass of extraembryonic somatopleuric mesoderm that connects the presumptive amnion to the inner wall of the cytotrophoblast at the embryonic pole of the blastocyst. During the second week of
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CH2 - Glossary of Key Terms and Concepts for Chapter 2...

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