ALIMENTARY CANAL. 



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 origi - 

 nal 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. 



Into the origin of the varying arrangements of the pancreatic 

 ducts it is not possible to enter in detail. In some cases, 

 e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a 

 division of the primitive gland and duct. In other cases, e.g. the 

 Bird, a second diverticulum springs from the alimentary tract. 

 In a large number of instances the primitive condition with a 

 single duct is retained. 



Postanal section of the mesenteron. In the embryos of 

 all the Chordata there is a section of the mesenteron placed 

 behind the anus. This section invariably atrophies at a com- 

 paratively early period of embryonic life ; but it is much better 

 developed in the lower forms than in the higher. At its 

 posterior extremity it is primitively continuous with the neural 

 tube (fig. 420), as was first shewn by Kowalevsky. 



The canal connecting the neural and alimentary canals has 

 already been described as the neurenteric canal, and represents 

 the remains of the blastopore. 



In the Tunicata the section of the mesenteron, which in all probability 

 corresponds to the postanal gut of the Vertebrata, is that immediately 



492 



