57G ' EMBBTOIiOGY. 



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

 only. 



The second factor is the separation of the liver- from, the prvmary 

 diaphragm, with which it was v/nited to form the septum transversvm,. 

 This takes place as follows : At the margin of the liver the peritoneumj. 

 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 hepatiSf 



-s/>. c. 



T\%. 315. — Gross section through an advanced embryo of a Babbit, to show how the pericardial 



cavity becomes surrounded by the pleural cavities, from Balfour. 

 lit, Heai-t ; ^c, pericardial cavity ; 'pl,'^, thoracic or pleural cavity ; Ig^ lung ; a?, alimentary 



canal ; ao, dorsal aorta ; cA, 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- 



