I during the fourth week of development a period of rapid growth the embryo begins to change shape from a flat tri laminar disc into a cylinder a process known as embryonic folding embryonic folding occurs in two planes the horizontal plane and the median plane and is the result of differing rates of growth of the embryonic structures folding of the embryo in the horizontal plane results in the development of two lateral body folds folding in the median plane results in the development of the cranial and caudal folds folding in both of these planes takes place simultaneously
resulting in the rapid development of the embryo the cylinder consists of three layers derived from the Tri laminar embryonic disc these are the endoderm the innermost layer the ectoderm the outermost layer and the mesoderm located in between the endoderm of the tri laminar disc is mainly responsible for the formation of the gastrointestinal tract as embryonic folding continues the endoderm moves towards the midline and fuses incorporating the dorsal part of the yolk sac to create the primitive gut tube the primitive gut tube differentiates into three main parts the foregut mid gut and hind gut the foregut
can be seen at the cranial or head end of the embryo it is temporarily closed by the oropharyngeal membrane which at the end of the fourth week of development ruptures to form the mouth the mid gut lies between the fore and hind gut and remains connected to the yolk sac until the fifth week of development as embryonic folding continues the connection to the yolk sac narrows into a stalk known as the vital induct the hindgut lies at the caudal or tail end of the embryo it is temporarily closed by the cloaca membrane which during the
seventh week of development ruptures to form the urogenital and anal openings as a result of embryonic folding the major body plan is established and the three germ layers continue to differentiate giving rise to their own specific tissues and organ systems you
