V.] THE LIVER. 131 



a common tube opening on the under surface of the oesophagus. This tube is 

 the trachea. 



At the end of each of the primary diverticula is a small vesicle which may 

 be called the primary lung-vesicle. It appears ultimately to become the abdo- 

 minal air-sac. 



The mesoblast round the primary diverticula becomes greatly thickened. 

 Into it secondary diverticula, still lined by hypoblast, penetrate; these again 

 give rise to tertiary branches and thus the bronchial tubes are formed. At right 

 angles'to the finest of these the arborescent branches so characteristic of the avian 

 lung are given off. The whole pulmonary structure is therefore the result of the 

 growth by budding of a system of branched hypoblastic tubes in the midst of a 

 mass of mesoblastic tissue, the hypoblastic elements giving rise to the epithelium 

 of the tubes and the mesoblast providing the elastic, muscular, cartilaginous, 

 connective and other tissues of the tracheal and bronchial walls. 



The air-sacs are primarily the dilated ends of the primitive diverticula or of 

 their main branches. At first there are three air-sacs on each side : one abdo- 

 minal (the end of the primitive diverticulum), another thoracic, and a third 

 extra-thoracic. An additional thoracic, and an additional extra -thoracic, making 

 in all five air-sacs on each side, appear at a later period. 



The pulmonary blood-vessels penetrate into the mesoblast surrounding the 

 bronchial tubes on about the iith day. 



19. The liver is the first formed chylopoietic appendage 

 of the digestive canal, and arises between the 55th and 60th 

 hour as a couple of diverticula one from either side of the 

 duodenum immediately behind the stomach (Fig. 42, I.). 

 These diverticula, though composed of both hypoblast and 

 mesoblast, are, according to Gotte, in the first instance solid, 

 and only subsequently become hollow. The right one is, 

 in all cases, from the first longer, but of smaller diameter 

 than the left. Situated a little behind the heart, they 

 embrace between them the meatus venosus or united trunk 

 of the omphalo-mesaraic veins, which as it passes between 

 them exhibits numerous small bulgings. As yet the vein 

 arid the diverticula, though in close contact, are not connected. 



Towards the end of the third day there may be observed 

 in the greatly thickened mesoblastic investment of either 

 diverticulum a number of cylindrical solid aggregations of 

 cells connected with, and apparently outgrowths from, the 

 hypoblast of the diverticulum. These cylinders rapidly 

 increase in number, apparently by division, their somewhat 

 swollen peripheral extremities come into contact and unite. 

 And thus, about the ninetieth hour, a sort of network of solid 

 thick strings of hypoblastic cells is formed, the mesoblast in 

 the meshes of the network becoming at the same time largely 

 converted into blood-vessels. In addition to this network of 

 solid hypoblastic cylinders, the diverticula also send out 



92 



