THE LUNGS AND THE LIVEE. 568 



cartilages of the second branchial arches, though KoUiker believes 

 it to arise independently. 



The lungs, as they grow backwards, project into the dorsal 

 part of the body cavity, pushing before them the peritoneal 

 lining of the cavity, which forms their pleural covering. At a 

 later stage, the portions of the body cavity into which the lungs 

 hang are shut oif, by the diaphragm and pericardium, from the 

 rest of the cavity, and become the definite pleural sacs. 



10. The Liver. 



The liver is present on the fifteenth day (Fig. 232, w) as 

 a short, hoUow diverticulum, with a compact mass of cells at 

 its blind end, arising from the ventral wall of the fore-gut and 

 the anterior wall of the yolk-stalk, immediately behind the heart. 



By the end of the third week (Pig. 215, w) the liver is of 

 larger size, and the bile-duct, or wide tubular passage connecting 

 the liver with the gut, is longer than before, but otherwise the 

 relations are much the same as in the earlier stage. 



During the fourth week the liver enlarges very rapidly 

 ■(Figs. 216, 243, w). It consists of a close network of anastomos- 

 ing epithelial cylinders, the development of which has not been 

 followed accurately : the meshes of the network are chiefly occu- 

 pied by blood-vessels, which are present in large numbers, and of 

 great size. The development and relations of these blood- vessels 

 •of the liver will be described in the next section of this chapter. 



The rapid growth of the liver continues during the succeeding 

 weeks. In the second half of gestation it is rather less marked 

 in proportion to the other viscera, but even at the end of preg- 

 nancy the weight of the liver is to that of the whole embryo as 1 

 to 18, while in the adult it is only 1 to 36. After birth the liver 

 diminishes rapidly, both in size and weight, owing to the cutting 

 •off of the blood supply previously brought to it by the allantoic 

 veias. 



The bile-duct rapidly lengthens during the fourth week ; 

 and the gall-bladder appears, as a diverticulum of the bile-duct, 

 before the end of the fifth week (Fig. 236, g.b). 



The large size of the liver during almost the whole period of 

 gestation, and its abundant vascular supply, indicate that it must 

 be of great physiological importance. It probably serves to 

 modify in some way the nutrient material brought from the 



o o 2 



