l88 THE ENTODERMAL CANAL AND THE BODY CAVITIES 



At the same time the hver 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) : (1) its ventral pericardial portion from the septum trans- 

 versum; 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 dia- 

 phragm, according to Bardeen, takes its origin from a pair of premuscle masses 

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



! umbilical vein 



Left umbilical vein 



Ventral mcscnlrry 



Right lobe of livci 



Lesser peritoneal sik 



Plica veruB cava 



Dorsal aorla 



Neural tuhr 



Ectoderm of body wail 



Left lobe of liver 

 Ventral mesentery 

 Duodenum 



Dorsal mesentery 

 Left posterior cardinal vein 



Nulochord 



Fig. 199. — Diagrammatic model of an embryo of 7 to 9 mm. showing the position of the lesser peri- 

 toneal 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 repre- 

 sent 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, Johns Hopkins Hospital Report, 

 vol. 9, 1900). 



Keith (Jour. Anat. and Physiol., vol. 39, 1905) derives the muscle of the diaphragm also 

 from the rectus and transversalis muscles of the abdominal wall. 



The cavities of the mesodermic 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. 



