454 



STRUCTURE OF VERTEBRA TA. 



the lungs lie ventrally and are double, though connected 



with the gullet by a single tube. 



The beginning of the intestine gives origin to the liver, 



which regulates the composition of the blood and secretes 



bile, and to the pancreas, which secretes digestive juices. 



The pancreas has often a multiple rudiment. 



From the hindmost region of the gut, the allantois 



grows out in all animals from Amphibians onwards. In 



Amphibians it is represented by a cloacal bladder ; in the 



higher Vertebrates it is a vas- 

 cular fcetal membrane concerned 

 with the respiration or nutrition 

 of the embryo, or both. 



Cilia are very common on the 

 lining of the intestine in Inverte- 

 brates, but they are much rarer 

 in Vertebrates. Yet as they 

 occur in Amphioxus, lampreys, 

 many fishes, Protopterus, some 

 Amphibians, and in embryonic 

 Mammals, it seems not unlikely 

 that the alimentary tract was 

 originally a ciliated tube. 



It is often said that, in some cases 

 at least, as in lamprey, frog, and newt, 

 the blastopore or opening of the 

 primitive gastrula cavity persists as the 

 anus of the adult ; but it seems doubt- 

 ful whether the anus is not always a 

 new formation. In many cases, at 

 least, an ectodermic invagination or 

 proctodeum meets the closed archenteron, and at the junction the two 

 epithelial layers give way, so that an open tube is formed. 



The formation of the anus does not take place close to the posterior 

 end of the primitive gut, but at a point some short distance in front of 

 this. In consequence the so-called post-anal gut is formed. This is 

 continuous with the neurenteric canal, and so communicates with the 

 neural canal. The post-anal gut attains in Elasmobranchs a relatively 

 considerable length. It has been very frequently found in Vertebrates, 

 and is probably of universal occurrence. After a longer or shorter 

 period it becomes completely atrophied, and with it the communication 

 between neural and alimentary canals is completely destroyed. 



Speculative. — The primitive gut was probably a smooth straight 

 tube, but the rapid multiplication of well-nourished cells would tend 

 to its increase in diameter and in length. But on increase in both 



Fig. 



liver, and pancreas 

 chick. — After Goette. 



The mesoderm is shaded ; the endo- 



derm dark. 

 lg., One of the lungs; St., stomach ; 



/., liver ; p., pancreas. 



