234 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
liver, but it may be immersed in the substance of the gland. In some 
cases, even in mammals, the gall bladder may be lacking. When a 
gall bladder is present, three regions may be recognized in: the bile 
ducts. Those parts which lead from the liver to the connexion with 
the bladder are called hepatic ducts; these are met by the cystic duct 
leading from the bladder, and the common duct, formed by the two and 
which empties into the intestine is the choledochal duct (fig. 240). 
The shape of the gland is in part determined by the shape of the body, 
being long in elongate species, sometimes consisting of two consecutive 
lobes. Another modifying factor is the shape and size of the adjacent 
organs, stomach, etc. Usually the liver is divided into right and left 
halves, these corresponding to the first division of the anlage, but these 
halves are hardly indicated in some of the teleosts. Frequently, and 
especially in mammals, the halves become subdivided into lobes of 
varying size, which are arranged in various ways. The liver is rela- 
tively larger in the ichthyopsida than in the amniotes, but the cyclo- 
stomes have a small liver, that of the myxinoids being in two parts. It 
is larger, too, in the flesh-eating than in the herbivorous species. The 
blood supply, chiefly through the portal vein and to a less extent by the 
hepatic. artery (see circulation) is very large. The color of the gland 
_is very variable, especially in teleosts, where it may be brown, yellow, 
purple, green and even vermilion. 
THE PANCREAS. 
The second largest of the digestive glands, the pancreas, secretes 
digestive ferments of great strength (trypsin, steapsin, amylopsin), 
which digest both proteids and carbohydrates. In some respects it 
resembles the salivary glands and so compensates in part for the. 
absence of them in the lower vertebrates (p. 220). The pancreas 
arises by diverticula from the wall of the intestine close to the liver. 
There are usually three of these diverticula, one dorsal and two ventral, 
the ventral soon uniting (fig. 242), but in the sharks there is only a 
single dorsal, diverticulum, while in the sturgeon there are two dorsal and 
twoventral. Inageneral way these develop much like the liver, the distal 
portions of the divisions forming the glands, which are of the acinous 
type; the proximal portions form the ducts. Of these ducts all may 
persist; all but one may disappear, while in the lampreys all may be 
lost. In many mammals two ducts persist, the ventral forming the 
