570 



EMBRYOLOGY. 



tha thorax (st) and below with the diaphragm for a little distance- 

 only. 



The second factor is the separation of the liver from the jyrimari^ 

 diaphragm, with which it was united to form the septum transversum^ 

 This takes place as follows : At the margin of the liver the peritoneum r 

 which originally covered only its under surface, grows over on to 

 its upper surface, separating it from the primary diaphragm. A 

 connection is retained near the wall of the trunk only. Thus is. 

 explained the development of the liy amentum coronarium hepatis*. 



Fig. 315. Cross section through an advanced embryo of a Rabbit, to show how the pericardial 



cavity becomes surrounded by the pleural cavities, from BALFOUR. 

 ht, Heart ; pc, pericardial cavity ; pl.p, thoracic or pleural cavity ; Ig, lung ; al, alimentary 



canal ; ao, dorsal aorta ; ch, chorda ; rp, rib ; st, sternum ; sp.c, spinal cord. 



which was disregarded in the section which treated of the ligamentous- 

 supports of the liver (p. 330). 



The diaphragm finally acquires its permanent condition by the 

 ingrowth of muscles from the wall of the trunk into the connective- 

 tissue lamella. 



(c) The Metamorphoses of the Arterial System. 



The development of the large arterial trunks lying in the vicinity 

 of the heart is of great interest from a comparative-anatomical point 

 of view. As in all Vertebrates at least five pairs of visceral arches- 



