570 



EMBRYOLOGY. 



the thorax (st} and below with tho diaphragm for a little distance 

 only. 



The, second factor is the separation of the liver from the primary 

 diaphragm, with ivhich it was united to form the septum transversum. 

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

 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 ligamentum coronarium hepatis, 



Z ^-^ xi? s^BP=^-, 



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 ; no, dorsal aorta ; ch. chorda ; r'p, 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 



