THE SMALL INTESTINE. 341 



have yet to distinguish differentiate later in a very simple 

 way; these are the gall-intestine (duodenum), which is 

 next to the stomach, the long empty intestine (jejunum) 

 which succeeds, and the last section of the small intestine, 

 the crooked intestine (ileum). 



The two large glands which we have already named, the 

 liver and the ventral salivary gland, grow out, as protuber- 

 ances, from the gall-intestine, or duodenum. The liver first 

 appears in the form of two small sacs, situated right and left 

 just behind the stomach (Figs. 284, /, 285, c). In many low 

 Vertebrates the two livers remain quite separate for a long 

 time (in the Myxinoides for life), and coalesce only imper- 

 fectly. In higher Vertebrates, on the other hand, the two 

 livers coalesce more or less completely at an early period, 

 and constitute one large organ. The intestinal-glandular 

 layer, which lines the hollow, pouch-like rudiment of the 

 liver, sends a number of branched processes into the investing 

 intestinal-fibrous layer; as these solid processes (rows of 

 gland-cells) again branch out, and as their branches coalesce, 

 the peculiar netted structure of the developed liver is 

 produced. The liver-cells, as the secreting organs which 

 form the bile, all originate from the intestinal-glandular 

 layer. The fibrous mass of connective tissue, which joins 

 this great cellular network into a large compact organ, and 

 which invests the whole, comes, on the other hand, from the 

 intestinal-fibrous layer. From the latter originate also the 

 great blood-vessels which traverse the entire liver, and 

 the innumerable netted branches of which are interlaced 

 with the network of the liver-cells. The gall-ducts, which 

 traverse the entire liver, collecting the bile and discharging 

 it into the intestine, originate as intercellular passages along 



