PERICARDIUM, PLEUROPERITOXEUM, DIAPHRAGM AND MESENTERIES. 375 



the separation between the pericardial cavity and the pleural cavities, the latter 

 for a time remain in open communication with the rest of the ccelom or peritoneal 

 cavity. The lungs, as they develop, grow into the pleural cavities (Fig. 332) 

 until their tips finally touch the cephalic surface of the liver. At this point 

 folds grow from the lateral and dorsal body walls (Fig. 333) and unite ventrally 

 with the primary diaphragm and medially with the mesentery. These folds 

 the pleuroperitoneal membranes separate the pleural cavities from the perit- 

 oneal cavity and complete the diaphragm. Thus the diaphragm, from the stand- 



a- PL cav. 



p.m. 



PC. cav. 



Lv.c. 



FIG. 335. 



FIG. 334. Transverse section through the thoracic region of a rabbit embryo of 15 days. Hochstetter. 

 FIG. 335. Transverse section through the thoracic region of a cat embryo of 25 mm. Hochstetter. 

 I.v.c.. Inferior vena cava; Inf.-c. 1., infracardiac lobe of lung; L. t lung; Oe.. oesophagus; PC. cav., 



pericardial cavity; PI. cav., pleural cavity; Pl.-p. m., pleuro- pericardial membrane; Pu.-h. r. t 



pulmo-hepatic recess. 



point of development, consists of two parts : a ventral part which is the cephalic 

 portion of the original septum transversum, and a dorsal part which develops 

 later from the body wall and is the closing membrane between the peritoneal 

 and pleural cavities. The musculature of the diaphragm is considered in the 

 chapter on the muscular system (p. 300). 



While the foregoing structures are being formed, decided changes take place 

 in their positions and relations. At first the heart lies far forward in the cervi- 

 cal region near the visceral arches. Later it migrates caudally and the pericardial 



