196 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



from (4) the dorsal mesentery. In addition to these, the striated muscle of the 

 diaphragm, according to Bardeen, takes its origin from a pair of pre-muscle 

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

 segment. This is the level at which the phrenic nerve enters the septum trans- 



A 



Pkuro-pericardiat canal 



Lung 



Peri cardiaL cavity 



Pleural cavity 

 Pleura -peritoneal membrane 



Luncj 



'Pleural cavity 

 \Pertcardicd 

 Heart rnembrane 



FIG. 190. Diagrams showing the development of the lungs and the formation of the pericardial mem- 

 brane (modified after Robinson). A, coronal section; B, transverse section. 



FIG. 191. Diagram showing the origin of the diaphragm (after Broman). i, septum transver- 

 sum; 2, 3, derivatives of mesentery; 4, 4, derivatives of pleuro-peritoneal membrane; 5, 5, parts de- 

 rived from the body walls. 



versum. 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, according to Bardeen. 



Keith 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 ccelom but in 



