Fig. 422. Diagram of the digestive tract of a Chick upon the fourth day. (After Götte.)
The black line indicates the hypoblast. The shaded part around it is the splanchnic mesoblast.
lg. lung; st. stomach; p. pancreas; l. liver.
The biliary ducts are formed either from some of the primitive hepatic cylinders, or, as would seem to be the case in Elasmobranchii and Birds ([fig. 422]), from the larger diverticula of the two primitive outgrowths.
The gall-bladder is so inconstant, and the arrangement of the ducts opening into the intestine so variable, that no general statements can be made about them. In Elasmobranchii the primitive median diverticulum ([fig. 421]) gives rise to the ductus choledochus. Its anterior end dilates to form a gall-bladder.
In the Rabbit a ductus choledochus is formed by a diverticulum from the intestine at the point of insertion of the two primitive lobes. The gall-bladder arises as a diverticulum of the right primitive lobe.
The liver is relatively very large during embryonic life and has, no doubt, important functions in connection with the circulation.
The pancreas. So far as is known the development of the pancreas takes place on a very constant type throughout the series of craniate Vertebrata, though absent in some of the Teleostean fishes and Cyclostomata, and very much reduced in most Teleostei and in Petromyzon.
It arises nearly at the same time as the liver in the form of a hollow outgrowth from the dorsal side of the intestine nearly opposite but slightly behind the hepatic outgrowth ([fig. 422], p). It soon assumes, in Elasmobranchii and Mammalia, somewhat the form of an inverted funnel, and from the expanded dorsal part of the funnel there grow out numerous hollow diverticula into the passive splanchnic mesoblast.
As the ductules grow longer and become branched, vascular processes grow in between them, and the whole forms a compact glandular body in the mesentery on the dorsal side of the alimentary tract. The funnel-shaped receptacle loses its original form, and elongating, assumes the character of a duct.
From the above mode of development it is clear that the glandular cells of the pancreas are derived from the hypoblast.