THE BODY CAVITIES. DIAPHRAGM AND MESENTERIES 



189 



At the same time the liver grows enormously, and on either side a 

 portion of the body wall is taken up into the septum transversum and 

 pleuro-peritoneal membranes. The diaphragm, according to Broman, 

 is thus derived from four sources (Fig. 198): (i) its ventral pericardial 

 portion from the septum transversum; its lateral portions from (2) the 

 pleuro-peritoneal membranes, plus (3) derivatives from the body wall; 

 lastly, a median dorsal portion is formed from (4) the dorsal mesentery. 

 In addition to these, the striated muscle of the diaphragm, according 

 to Bardeen (1900), takes its origin from a pair of premuscle masses which 

 in 9 mm. embryos lie one on each side opposite the fifth cervical segment. 



Right umbilical vein 

 Ventral mesentery 



Left umbilical vein 



Right lobe of liver 



Lesser peritoneal sac 



Plica vena cavtz 



Ectoderm of body wall 



Left lobe of liver 

 Ventral mesentery 

 Duodenum 



Dorsal mesentery 

 Left posterior cardinal vein 



Dorsal aorta 



Neural tube 



- Notochord 



FIG. 199. Diagrammatic model of an embryo of 7 to 9 mm., showing the position of the 

 lesser peritoneal sac. The embryo is represented as sectioned transversely, caudal to the liver, 

 so that one looks at the caudal surface of the section and of the liver, and cranially into the 

 body cavities. 



This is the level at which the phrenic nerve enters the septum transversum. 

 The exact origin of these muscle masses is in doubt, but they probably 

 represent portions of the cervical myotomes of this region. The muscle 

 masses migrate caudally with the septum transversum and develop chiefly 

 in the dorsal portion of the diaphragm (Bardeen, 1900). 



The cavities of the mesodermal segments are regarded as portions of the coelom, but 

 in man they disappear early. The development of the vaginal sacs which grow out from 

 the inguinal region of the peritoneal cavity into the scrotum will be described in Chapter 

 VIII. 



