352 TEXT-BOOK OF EMBRYOLOGY. 



the original common mesentery, since the caecum and appendix together are 

 formed as an evagination from the primitive gut. 



Normally the mesentery of the small intestine forms no secondary attach- 

 ments, but is thrown into a number of folds which correspond to the coils of the 

 intestine. 



The secondary attachments of the greater omentum and the fusion of the 

 two lamellae have been described earlier in this chapter (p. 348) . The mesen- 

 teries of the urogenital organs are considered in connection with the develop- 

 ment of those organs (Chapter XV). 



The Peritoneum. The thin layer of tissue composed of delicate fibrous 

 connective tissue and mesothelium which lines the abdominal cavity and is re- 

 flected over the various omenta, mesenteries and visceral organs, is derived 

 wholly from the mesoderm. The lining of the ccelom is composed of mesothe- 

 lium and mesenchyme. The latter gives rise to the connective tissue of the 

 serous membranes, and the mesothelial layer becomes the mesothelium of these 

 membranes. 



Anomalies. 



THE PERICARDIUM. Anomalous conditions of the pericardium are usually, 

 although not necessarily, associated with anomalies of the heart. They may 

 also be associated with defects in the diaphragm. Displacement of the heart 

 (ectopia cordis) is accompanied by displacement of the pericardium. The 

 heart sometimes protrudes through the thoracic wall, and, as a rule, in such cases 

 is covered by the protruding pericardium. In extensive cleft of the thoracic 

 wall (thoracoschisis, Chap. XX) the pericardium may be ruptured. 



THE DIAPHRAGM. The most common malformation of the diaphragm is a 

 defect in its dorsal part, occurring much more frequently on the left than on the 

 right side. The defect may affect but a small portion or may be extensive, the 

 peritoneum being directly continuous with the parietal layer of the pleura. 

 Such defects are due to the imperfect development of the pleuro-peritoneal mem- 

 brane which normally grows from the dorso-lateral part of the body wall and 

 fuses with the edge of the primary diaphragm, thus separating the pleural and 

 and peritoneal cavities (p. 345) . The most conspicuous result of defects in the 

 dorsal part of the diaphragm is diaphragmatic hernia, in which parts of the 

 stomach, liver, spleen and intestine project into the pleural cavity, either free or 

 enclosed in a peritoneal sac. Defects in the ventral part of the diaphragm, due 

 to imperfect development of portions of the septum transversum, are not 

 common, 



THE MESENTERIES AND OMENTA. Extensive malformations of the mesen- 

 teries apparently do not occur without extensive malformations of the digestive 

 tract. One of the most striking anomalous conditions is a retained embryonic 



