Folding+of+the+Embryo,+Basic+Body+Form,+Coelom+Formation

=**Objectives**=


 * 1. Describe the results of the lateral and longitudinal folding processes that occur in the embryo.**

The growth rate at the sides of the embryonic disk fails to keep pace with the rapid growth of the long axis as the embryo increases rapidly in length. This results in the folding at the cranial and caudal ends of the embryo and relative constriction at the junction of the embryo and the yolk sac.

By the beginning of the fourth week, the neural folds in the cranial region have thickened to form the primordium of the brain. Initially, the brain projects dorsally into the amniotic cavity. Later, the developing forebrain grows cranially beyond the oropharyngeal membrane and overhangs the developing heart. Concomitantly, the septum transfersum, primordial heart, pericardial coelom, and oropharyngeal membrane move to the ventral surface of the embryo. Part of the yolk sac is incorporated into the embyro as the foregut. After folding, the septum transfersum lies caudal to the heart where it subsequently develps into the central tendon of the diaphram.

Folding of the caudal end results in the partial incorporation of the endodermal germ layer into the embryo as the hindgut. The terminal part of the hindgut dilates and form the coaca. Before folding, the primative streak lies cranial to the coacal membrane; after foldling, it lies caudal. The connecting stalk becomes attached to the ventral surface and the allantois is partially incorporated into the embryo.


 * 2. Define: coelom, mesentery, septum transversum.**

__Coelom__

__Mesentary__

__Septum Transversum__


 * 3. Describe/define the adult structures dervived from teh primitive intraembryonic coelom.**


 * 4. Describe/define the origin of the cells lining the intraembryonic coelom and its definitive derivatives.**


 * 5. Describe the configuration of the intraembryonic coelom following the development of the head fold.**


 * 6. Describe/define the pleuropericardial membranes; the pleuroperitoneal membranes.**


 * 7. Describe the relative position of the (thoracoabdominal) diaphram during its development**


 * 8. List the four major tissue sources contributing to the definitive diaphragm. Correlate each with the related area of the definitive diaphragm.**


 * 9. Describe the probable developmental errors that produce: congenital posteriolateral diaphramatic hernia, congenital hiatal hernia, congenital retrosternal hernia.**